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REASONING · PUNJAB POLICE SI SERIES

Clocks and Calendars for Punjab Police SI

All 33 clock and calendar questions the Punjab Police SI has asked since 2016: odd days, the leap-year rule including the century exception, finding the day of the week across years, when a calendar repeats, and the angle between the hands, with every question solved.

Punjab PolicePSSSBPunjab Police SI13 MIN READ

Clocks and calendars are 33 of the 692 reasoning questions the Punjab Police SI has asked, and the split is lopsided: 31 are calendars and only 2 are clocks. If your preparation time is limited, spend it on the calendar. The calendar has exactly one idea behind it, called the odd day. A week is seven days, so only the remainder after dividing by seven can change the day of the week. An ordinary year is 365 days, and 365 ÷ 7 leaves 1 — so the same date next year falls one weekday later. A leap year leaves 2. That single fact, applied carefully, answers thirty-one of these thirty-three questions. Nothing here needs memorising beyond the leap-year rule and the lengths of the months. Everything else is division by seven.

HOW LONG THIS REALLY TAKES

The teaching part above and below — the explanations, the figures and the worked examples on screen — is about 13 minutes. The collapsed question banks hold 33 solved past questions in all; working through every one of them takes roughly 62 minutes. Do that across several sittings, not in one go.

33
SI questions
all solved below
18
day of the week
count the odd days
13
repeating calendars
same kind, odd days zero
2
clock angles
one formula

Part 1 · Odd days, and the leap-year rule

The one idea the whole calendar rests on

Odd days for the four periods that matter
Every calendar question reduces to counting odd days between two dates and stepping the weekday forward by that many.
KEY TERM
A leap year

A year divisible by 4 is a leap year — except a century year, which must be divisible by 400. So 1996, 2012 and 2024 are leap years; 2000 is a leap year because 2000 ÷ 400 = 5; but 1900 and 2100 are not, even though both divide by 4. The SI has set this exception directly, and candidates who know only the “divisible by four” half of the rule get it wrong.

MonthDaysOdd days
Jan, Mar, May, Jul, Aug, Oct, Dec313
Apr, Jun, Sep, Nov302
February, ordinary year280
February, leap year291
Odd days are just the remainder on division by seven. February in an ordinary year adds nothing at all to the weekday, which is why dates either side of it so often share a day.

Part 2 · The day of the week — 18 questions

Jump whole years first, then move within the year

Half of these give you a day and a date and ask for the day on another date. Do it in two steps, never one. Step 1 — jump whole years. From 19 July 2000 to 19 July 2013 is thirteen years, so start with 13. Then add 1 more for every 29 February you actually cross between the two dates — not for every leap year on the calendar. The two are not the same: 2000 is a leap year, but 29 February 2000 falls before 19 July 2000, so you never cross it. Crossing 29 Feb in 2004, 2008 and 2012 gives 3, so 13 + 3 = 16. Reduce by sevens: 16 = 2×7 + 2, so 2 odd days. That moves the same date to the right weekday in the later year. Step 2 — move within the year. Now count the days from that date to the target date, month by month, and reduce by sevens again. The other half are shorter: if the 1st is a Sunday, how many Sundays in the month, or the day before yesterday was Monday, so what is tomorrow. For those, anchor on the day you are given and step one day at a time on paper. Phrases like “the day before yesterday” are where the marks are lost, not the arithmetic.

QUICK CHECK
If 19 July 2000 is a Wednesday, then what will be the day of the week on 28 May 2013? (SI 15 Jul 2023)
Tap an option to answer
QUICK CHECK
If every Saturday and Sunday is a holiday, how many working days will there be in a month that begins on a Sunday and has 31 days? (SI 2 Jul 2023)
Tap an option to answer
Every other day-of-the-week question (16)Tap to open
CHECK YOURSELF

Assuming 1^{st} January, 2004 was Monday, then what day of the week would be 16^{th} September, 2004? (SI 18 Aug 2021)

  1. 1Saturday
  2. 2Wednesday
  3. 3Monday
  4. 4Tuesday
Show answer

Answer: 3 — Monday

Count the days elapsed from 1 January and reduce modulo 7. 2004 is a leap year (divisible by 4 and not a century), so February has 29 days. Days completed before September: January 31 + February 29 + March 31 + April 30 + May 31 + June 30 + July 31 + August 31 = 244. Adding 16 makes 16 September the 260th day of the year. The number of days after 1 January is 260 − 1 = 259, and 259 = 7 × 37 exactly, so there are 0 odd days. 16 September therefore falls on the same weekday as 1 January, namely Monday. Anyone who forgets the leap day gets 258 days, one odd day short, and lands on Sunday; taking February as 28 and also miscounting a 30-day month is what produces the Saturday and Tuesday distractors.

CHECK YOURSELF

The third day of a month is a Tuesday. Which day of the week will the 29th of that month be? (SI 19 Aug 2021)

  1. 1Saturday
  2. 2Monday
  3. 3Sunday
  4. 4Tuesday
Show answer

Answer: 3 — Sunday

Days of the week repeat every 7 days, so only the remainder on division by 7 matters. From the 3rd to the 29th is 29 − 3 = 26 days. 26 = 3 × 7 + 5, so the 29th falls 5 weekdays after Tuesday: Wednesday, Thursday, Friday, Saturday, Sunday. The 29th is therefore a Sunday. A quick check: the 3rd, 10th, 17th and 24th are all Tuesdays, and 29 is five days past the 24th — Sunday again. Option 4 (Tuesday) is the trap for anyone who assumes 26 is a multiple of 7, and option 1 (Saturday) comes from counting 4 days instead of 5.

CHECK YOURSELF

If the day before yesterday was Monday, then what will be the day after tomorrow? (SI 20 Jun 2023)

  1. 1Friday
  2. 2Monday
  3. 3Sunday
  4. 4Tuesday
Show answer

Answer: 1 — Friday

The day before yesterday is two days back from today, so today = Monday + 2 = Wednesday. The day after tomorrow is two days forward from today, so it is Wednesday + 2 = Friday. In short, the day after tomorrow is always four days after the day before yesterday. Option 3 (Sunday) is the trap for counting backwards instead of forwards.

CHECK YOURSELF

2 January 2002 was which day of the week? (SI 21 Jun 2023)

  1. 1Thursday
  2. 2Wednesday
  3. 3Monday
  4. 4Tuesday
Show answer

Answer: 2 — Wednesday

Count odd days up to the date. Take 2001 complete years plus 2 days of January 2002. 1600 years give 0 odd days and 400 years give 0 odd days, so 2000 years contribute 0. The single remaining ordinary year (2001) contributes 1 odd day. January 2002 contributes 2 days. Total odd days = 0 + 1 + 2 = 3, and counting 0 = Sunday, 1 = Monday, 2 = Tuesday, 3 = Wednesday. Cross-check the other way: 1 January 2000 was a Saturday, 2000 was a leap year (366 days = 2 odd days) so 1 January 2001 was a Monday, and 2001 was ordinary (1 odd day) so 1 January 2002 was a Tuesday — making 2 January 2002 a Wednesday. Tuesday (option 4) is the trap because it is the day 1 January fell on.

CHECK YOURSELF

If 3 December 2021 was a Friday, then what will be the day of the week on 3 January 2022? (SI 21 Jun 2023)

  1. 1Sunday
  2. 2Wednesday
  3. 3Tuesday
  4. 4Monday
Show answer

Answer: 4 — Monday

December has 31 days, so from 3 December 2021 to 3 January 2022 is exactly 31 days. 31 ÷ 7 leaves a remainder of 3, so the day advances by 3 from Friday. Friday + 1 = Saturday, + 2 = Sunday, + 3 = Monday. Sunday (option 1) is the trap for anyone who takes the remainder as 2 by counting 30 days instead of 31.

CHECK YOURSELF

Today is a Sunday. After 96 days, it will be a : (SI 29 Jun 2023)

  1. 1Saturday
  2. 2Friday
  3. 3Wednesday
  4. 4Thursday
Show answer

Answer: 2 — Friday

Days of the week repeat every 7 days, so only the remainder on division by 7 matters (the odd days). 96 ÷ 7 = 13 remainder 5, since 7 × 13 = 91 and 96 − 91 = 5. So the answer is 5 days after Sunday: Monday, Tuesday, Wednesday, Thursday, Friday. Counting 96 as 13 full weeks and mistakenly taking 6 odd days gives Saturday, the trap in option 1.

CHECK YOURSELF

If the day before yesterday was Friday, then what day of the week will the day after tomorrow be? (SI 1 Jul 2023)

  1. 1Tuesday
  2. 2Sunday
  3. 3Monday
  4. 4Wednesday
Show answer

Answer: 1 — Tuesday

Anchor on the stated day and step forward one day at a time. Day  ·  Which Day before yesterday  ·  Friday Yesterday  ·  Saturday Today  ·  Sunday Tomorrow  ·  Monday Day after tomorrow  ·  Tuesday The day after tomorrow is 4 days after Friday: Friday + 4 = Tuesday. Option 2 (Sunday) is the trap — that is today, not the day after tomorrow.

CHECK YOURSELF

Which of the following days of the week was 1 May 2002? (SI 2 Jul 2023)

  1. 1Friday
  2. 2Wednesday
  3. 3Tuesday
  4. 4Thursday
Show answer

Answer: 2 — Wednesday

Anchor on 1 January 2000, a Saturday. 2000 is a leap year (366 days = 2 odd days), so 1 Jan 2001 was Monday; 2001 is ordinary (1 odd day), so 1 Jan 2002 was Tuesday. From 1 Jan 2002 to 1 May 2002: Jan 31 + Feb 28 + Mar 31 + Apr 30 = 120 days, and 120 = 17×7 + 1, i.e. 1 odd day. Tuesday + 1 = Wednesday. Note 2002 is not a leap year, so February contributes 28, not 29 — using 29 gives Thursday (option 4), the trap.

CHECK YOURSELF

I went to the market 10 days ago. If today is a Tuesday, then what day was it when I went to the market? (SI 3 Jul 2023)

  1. 1Monday
  2. 2Saturday
  3. 3Sunday
  4. 4Tuesday
Show answer

Answer: 2 — Saturday

Weekdays repeat every 7 days, so only the remainder matters: 10 ÷ 7 leaves 3. Going back 3 days from Tuesday gives Monday, Sunday, Saturday — so the day was Saturday. The trap is to count 10 back one day at a time and stop early at Sunday, or to treat 10 days ago as 7 days ago and answer Tuesday.

CHECK YOURSELF

If 29 July 2010 was Thursday, then what was the day of the week on 29 July 2022? (SI 3 Jul 2023)

  1. 1Monday
  2. 2Saturday
  3. 3Friday
  4. 4Sunday
Show answer

Answer: 3 — Friday

Rule: an ordinary year moves a fixed date forward by 1 weekday and a leap year by 2, so count odd days as (number of years) + (number of 29 Februaries crossed). From 29 July 2010 to 29 July 2022 is 12 years. The 29 Februaries falling inside that window are 2012, 2016 and 2020 — three of them. Odd days = 12 + 3 = 15, and 15 ÷ 7 leaves a remainder of 1. Thursday + 1 day = Friday. The usual slip is to count 2024 as well, or to forget the leap days and answer Tuesday.

CHECK YOURSELF

If 18 July 2021 was a Sunday, what day of the week was 20 August 2021? (SI 9 Jul 2023)

  1. 1Sunday
  2. 2Thursday
  3. 3Friday
  4. 4Saturday
Show answer

Answer: 3 — Friday

Count the days from 18 July to 20 August 2021. July has 31 days, so 31 − 18 = 13 days take you to 31 July, and 20 more take you to 20 August: 13 + 20 = 33 days. Odd days = 33 ÷ 7 = 4 weeks and 5 days remaining, so the answer is 5 days past Sunday: Monday, Tuesday, Wednesday, Thursday, Friday. 20 August 2021 was a Friday. Thursday is the trap for anyone who counts 32 days (forgetting that 31 July itself is day 13), and Sunday for anyone who takes 35 days.

CHECK YOURSELF

If 1 January, 2021 was Friday, then what was the day of the week on 1 January, 2022? (SI 14 Jul 2023)

  1. 1Saturday
  2. 2Friday
  3. 3Thursday
  4. 4Sunday
Show answer

Answer: 1 — Saturday

The day of the week shifts forward by the number of odd days in the year that separates the two dates. The year in between is 2021, which is not divisible by 4, so it is an ordinary year of 365 days. 365 ÷ 7 leaves a remainder of 1, that is one odd day. So 1 January 2022 falls one weekday after 1 January 2021: Friday + 1 = Saturday. Option 4 (Sunday) is what you get if you wrongly treat 2021 as a leap year and add two days.

CHECK YOURSELF

If 13 April 2002 was a Saturday, then what was the day of the week on 9 March 2014? (SI 15 Jul 2023)

  1. 1Saturday
  2. 2Friday
  3. 3Sunday
  4. 4Monday
Show answer

Answer: 3 — Sunday

Step 1 — jump whole years. From 13 April 2002 to 13 April 2014 is 12 years containing the leap days of 2004, 2008 and 2012, so odd days = 12 + 3 = 15, and 15 ÷ 7 leaves 1. Saturday + 1 = Sunday, so 13 April 2014 was a Sunday. Step 2 — walk back to 9 March 2014. From 9 March to 13 April is 22 (rest of March) + 13 = 35 days, which is exactly 5 weeks, so the day does not change. Therefore 9 March 2014 was also a Sunday.

CHECK YOURSELF

What will be the day on 31 December 2012 if the day on 1 January 2012 is a Tuesday? (SI 29 Jul 2023)

  1. 1Saturday
  2. 2Wednesday
  3. 3Monday
  4. 4Tuesday
Show answer

Answer: 2 — Wednesday

2012 is a leap year, so it has 366 days, but from 1 January to 31 December only 365 days elapse. 365 ÷ 7 leaves a remainder of 1 odd day, so 31 December falls one weekday after 1 January: Tuesday + 1 = Wednesday. The trap is option 4 (Tuesday): a common year begins and ends on the same weekday, but 2012 is a leap year and the extra 29 February pushes the end of the year forward by one day.

CHECK YOURSELF

If 1 January 2011 was a Saturday, then what day of the week was it on 31 December 2011? (SI 31 Jul 2023)

  1. 1Saturday
  2. 2Monday
  3. 3Sunday
  4. 4Friday
Show answer

Answer: 1 — Saturday

2011 is not divisible by 4, so it is an ordinary year of 365 days. From 1 January to 31 December is 364 days, and 364 = 52 × 7 exactly, so there are no odd days in between. A whole number of weeks later the day repeats, so 31 December 2011 was also a Saturday. The usual slip is to add the single odd day of a 365-day year and answer Sunday (option 3) — but that odd day carries you to 1 January 2012, not to 31 December 2011.

CHECK YOURSELF

If 27 October 2011 was Thursday, then what was the day of the week on 23 September 2022? (SI 31 Jul 2023)

  1. 1Saturday
  2. 2Wednesday
  3. 3Sunday
  4. 4Friday
Show answer

Answer: 4 — Friday

Work in odd days. From 27 October 2011 to 27 October 2021 is 10 years containing three 29-Februaries (2012, 2016, 2020), so 10 × 365 + 3 = 3653 days; 3653 = 521 weeks + 6, so 27 October 2021 was Thursday + 6 = Wednesday. From 27 October 2021 to 23 September 2022 is 331 days (Nov 30 + Dec 31 + Jan 31 + Feb 28 + Mar 31 + Apr 30 + May 31 + Jun 30 + Jul 31 + Aug 31 + Sep 23 + 4 remaining days of October), and 331 = 47 weeks + 2, so the day is Wednesday + 2 = Friday. Checked directly: the gap is 3984 days = 569 weeks + 1, i.e. Thursday + 1 = Friday. The common slip is to treat the second stretch as a full year (+1 odd day) and land on Sunday (option 3).

Part 3 · When does a calendar repeat? — 13 questions

Two conditions, both of which must hold

Two years can share a calendar only if both conditions are satisfied: One — the same kind of year. A leap year can only repeat as a leap year, because its February has an extra day and every date after February would otherwise shift. Two — the odd days between them total a multiple of 7. Add 1 for each ordinary year and 2 for each leap year from the first year up to, but not including, the second. When that total divides exactly by seven, 1 January falls on the same weekday and the whole year matches. Here you may count leap years straight off the calendar, unlike Step 1 in Part 2. These spans run 1 January to 1 January, so every leap year in the list has its 29 February inside the span. When a span starts mid-year, count the 29 Februaries you actually cross instead. In practice an ordinary year repeats after 6 or 11 years and a leap year after 28, but the gap changes near a century year that is not a leap year, so count rather than quote the number. The related questions — how many days did this person live, which year is a leap year — use the same counting.

QUICK CHECK
How many leap years are there in the first 400 years of a calendar? (SI 31 Jul 2023)
Tap an option to answer
Every other calendar-repetition question (12)Tap to open
CHECK YOURSELF

Sadashiv remembers that Sanjay’s birthday is certainly after January 12 but not later than January 16. If Meher remembers that Sanjay’s birthday is before January 17 but not before January 13, on which of the following day was Sanjay’s birthday ? (SI 2016)

  1. 114
  2. 215
  3. 316
  4. 4Cannot be determined
Show answer

Answer: 4 — Cannot be determined

Turn each statement into a set of dates. Sadashiv: after January 12 but not later than January 16 gives {13, 14, 15, 16}. Meher: before January 17 but not before January 13 gives {13, 14, 15, 16} as well — the same four dates, so the second statement adds nothing to the first. Four dates survive the overlap, and one of them (13 January) is not even offered, so no single day can be fixed. Options 1, 2 and 3 each name just one of four equally possible dates, so none of them is forced by the information given.

CHECK YOURSELF

Assuming 1^{st} March 1999 was Thursday, what day was on 13^{th} February 1999? (SI 20 Aug 2021)

  1. 1Thursday
  2. 2Sunday
  3. 3Friday
  4. 4Tuesday
Show answer

Answer: 4 — Tuesday

1999 is not a leap year (1999 is not divisible by 4), so February 1999 had 28 days. Count forward from 13 February to 1 March: the rest of February is 28 − 13 = 15 days, plus 1 day to reach 1 March, giving 16 days in all. Divide by 7: 16 = 2 × 7 + 2, so 1 March falls 2 weekdays after 13 February. Going back two days from Thursday gives Wednesday, then Tuesday. So 13 February 1999 was a Tuesday. Option 1 (Thursday) is the trap for anyone who assumes the gap is a whole number of weeks; option 3 (Friday) comes from counting the two days forward instead of backward.

CHECK YOURSELF

It is Monday today. What day will be the next day after 2 days from today? (SI 16 Oct 2022)

  1. 1Sunday
  2. 2Thursday
  3. 3Monday
  4. 4Friday
Show answer

Answer: 2 — Thursday

Break the sentence into its two steps. "2 days from today" is one step; "the next day after" that is a second step. So the total is today + 2 + 1 = today + 3 days. Count from today  ·  Day today  ·  Monday +1  ·  Tuesday +2  ·  Wednesday — this is "2 days from today" +3  ·  Thursday — the next day after that So the answer is Thursday. Why the others fail: Wednesday, the answer to the shorter question "what is 2 days from today", is not even offered, which confirms that the extra step is intended; Monday would be today or a full week later; Friday is today + 4, one day too many; Sunday is the day before Monday.

CHECK YOURSELF

The calendar of the year 2005 was used before in the year ______. (SI 16 Oct 2022)

  1. 11993
  2. 21994
  3. 31998
  4. 41999
Show answer

Answer: 2 — 1994

Two years share a calendar only if both have the same leap status and both begin on the same weekday. 2005 is an ordinary year and 1 January 2005 was a Saturday. Count odd days backwards from 2005: an ordinary year shifts the weekday by 1 and a leap year by 2. 2004 (leap) 2, 2003 1, 2002 1, 2001 1, 2000 (leap) 2, 1999 1, 1998 1, 1997 1, 1996 (leap) 2, 1995 1, 1994 1 — running total from 1994 up to 2005 = 1+1+2+1+1+1+2+1+1+1+2 = 14, which is 0 (mod 7). So 1 January 1994 fell on the same weekday as 1 January 2005, and 1994 is also an ordinary year — the calendars match exactly. 1993 began on a Friday and 1999 on a Friday, and 1998 on a Thursday, so none of them repeats in 2005.

CHECK YOURSELF

If 1 January 2005 is a Monday, then which day will 29 January 2006 be? (SI 20 Jun 2023)

  1. 1Saturday
  2. 2Monday
  3. 3Tuesday
  4. 4Sunday
Show answer

Answer: 3 — Tuesday

Work in two steps: first move a whole year, then move within January. Step 1. 2005 is not a leap year (2005 is not divisible by 4), so it has 365 days = 52 weeks + 1 odd day. Moving from 1 January 2005 to 1 January 2006 therefore advances the weekday by 1: Monday + 1 = Tuesday. So 1 January 2006 is a Tuesday. Step 2. From 1 January to 29 January is 28 days, and 28 = 4 × 7, so it is a whole number of weeks and contributes 0 odd days. Hence 29 January 2006 falls on the same weekday as 1 January 2006 — Tuesday. Option 2 (Monday) is the trap for anyone who forgets the single odd day of 2005 and simply carries Monday forward.

CHECK YOURSELF

The calendar for the year 2009 will be the same for which of the following years? (SI 29 Jun 2023)

  1. 12015
  2. 22020
  3. 32016
  4. 42018
Show answer

Answer: 1 — 2015

Two years share a calendar when (a) both are of the same kind — both ordinary or both leap — and (b) the odd days between them add up to a multiple of 7. An ordinary year contributes 1 odd day, a leap year 2. Count odd days from 2009 up to the year before the repeat: 2009: 1, 2010: 1, 2011: 1, 2012: 2 (leap), 2013: 1, 2014: 1 → total 7 ≡ 0 (mod 7). So 2015 starts on the same weekday as 2009, and both 2009 and 2015 are ordinary years. The calendar repeats in 2015. Option 3 (2016) is a leap year and so cannot match an ordinary year's calendar at all; 2020 is also a leap year; by 2018 the running total of odd days is 10 ≡ 3, not 0.

CHECK YOURSELF

If 1 January 1996 was a Monday, how many Mondays were there in 1996? (SI 1 Jul 2023)

  1. 153
  2. 254
  3. 351
  4. 452
Show answer

Answer: 1 — 53

1996 is divisible by 4 and is not a century year, so it is a leap year: 366 days. 366 = 52 × 7 + 2. So every weekday occurs at least 52 times, and two weekdays occur a 53rd time — the two "odd days", which are the first two days of the year. 1 January 1996 = Monday, so 2 January 1996 = Tuesday. Monday and Tuesday are the two extra days, and each occurs 53 times; the other five weekdays occur 52 times. Mondays in 1996 = 53. Option 4 (52) is the count you get if you forget the leap year's second odd day.

CHECK YOURSELF

The calendar for which of the following years will look exactly the same as the calendar for the year 2007? (SI 9 Jul 2023)

  1. 12016
  2. 22017
  3. 32018
  4. 42014
Show answer

Answer: 3 — 2018

A calendar repeats when two conditions hold together: the odd days between the two years add up to a multiple of 7, and both years are of the same kind (both ordinary or both leap). 2007 is an ordinary year, so the answer must also be an ordinary year — which already rules out 2016 (leap). An ordinary year contributes 1 odd day and a leap year 2. Adding from 2007 onwards: Year  ·  Odd days  ·  Running total  ·  Total mod 7 2007  ·  1  ·  1  ·  1 2008  ·  2  ·  3  ·  3 2009  ·  1  ·  4  ·  4 2010  ·  1  ·  5  ·  5 2011  ·  1  ·  6  ·  6 2012  ·  2  ·  8  ·  1 2013  ·  1  ·  9  ·  2 2014  ·  1  ·  10  ·  3 2015  ·  1  ·  11  ·  4 2016  ·  2  ·  13  ·  6 2017  ·  1  ·  14  ·  0 The running total first becomes a multiple of 7 after 2017 is counted, so the calendar repeats in 2018, and 2018 is an ordinary year just like 2007. 2014 (total 3) and 2017 (total 6 at the start of 2017) are simply not clear of the 7.

CHECK YOURSELF

Sharda was born on 13 September 1972 and died on 24 March 2018. Find her exact age. (SI 14 Jul 2023)

  1. 165 y 6 m 30 d
  2. 235 y 2 m 12 d
  3. 349 y 7m 3 d
  4. 445 y 6 m 12 d
Show answer

Answer: 4 — 45 y 6 m 12 d

Subtract date of birth from date of death, column by column. Years: 2018 − 1972 = 46. Months: 3 − 9 is negative, so borrow a year: 46 − 1 = 45 years and 3 + 12 − 9 = 6 months. Days: 24 − 13 = 11. So from 13 September 1972 to 13 September 2017 is 45 years, from 13 September 2017 to 13 March 2018 is 6 months, and 13 March to 24 March is a further 11 days (12 if the day of death is counted as well, which is the count the paper has used). That is 45 y 6 m, which only option 4 carries. Option 3 (49 y) and option 1 (65 y) are far too large, and option 2 (35 y) far too small.

CHECK YOURSELF

Which year will have the same calendar as that of 2016? (SI 15 Jul 2023)

  1. 12040
  2. 22032
  3. 32024
  4. 42044
Show answer

Answer: 4 — 2044

A calendar repeats when the odd days since the reference year total a multiple of 7 and the leap/ordinary character matches. An ordinary year contributes 1 odd day and a leap year 2. 2016 is a leap year, and a leap year's calendar repeats after 28 years provided no non-leap century year (such as 2100) falls in between. 2016 + 28 = 2044, and 2100 is not crossed, so 2044 is the answer. Check the others by counting odd days from 2016: 2016 → 2024: 8 years with 2 leap years = 8 + 2 = 10 odd days ≡ 3 (mod 7). 2016 → 2032: 16 years with 4 leap years = 20 ≡ 6. 2016 → 2040: 24 years with 6 leap years = 30 ≡ 2. 2016 → 2044: 28 years with 7 leap years = 35 ≡ 0. ✓ 2024 is the trap because it is the next leap year, but being a leap year is not enough.

CHECK YOURSELF

What is the number of days from 18 January 2021 to 27 July 2021, including both days? (SI 29 Jul 2023)

  1. 1191
  2. 2189
  3. 3190
  4. 4192
Show answer

Answer: 1 — 191

Count month by month, remembering that 2021 is not a leap year so February has 28 days. January from the 18th to the 31st, inclusive, is 31 − 18 + 1 = 14 days; February 28; March 31; April 30; May 31; June 30; July up to the 27th is 27. Total = 14 + 28 + 31 + 30 + 31 + 30 + 27 = 191. Option 3 (190) is what you get by taking the plain difference of the two dates and forgetting to add back the starting day.

CHECK YOURSELF

Which of the following years will have the same calendar as that of year 2021? (SI 31 Jul 2023)

  1. 12026
  2. 22025
  3. 32027
  4. 42028
Show answer

Answer: 3 — 2027

A year repeats its calendar when the odd days after it total a multiple of 7. A common year gives 1 odd day, a leap year 2. Counting from 2021: 2021 gives 1, 2022 gives 1, 2023 gives 1, 2024 is a leap year and gives 2, 2025 gives 1, 2026 gives 1. The running totals are 1, 2, 3, 5, 6, 7 — the total first reaches 7 (0 mod 7) after 2026, so 2027 repeats the 2021 calendar. 2021 and 2027 both begin on a Friday and both are common years, which is what the repeat needs. Option 2 (2025) is only 4 odd days on.

Part 4 · Clocks — 2 questions, one formula

How far each hand has travelled

The minute hand sweeps 360° in 60 minutes, so it moves 6° a minute. The hour hand sweeps 360° in 12 hours, so it moves 30° an hour, which is 0.5° a minute — and it keeps moving while the minutes pass, which is the detail everyone forgets. Measuring both from 12 o'clock, at H hours and M minutes the hour hand is at 30H + 0.5M degrees and the minute hand is at 6M degrees. Subtracting gives the standard formula angle = | 30H − 5.5M |. If the result is more than 180°, subtract it from 360° to get the smaller angle, which is what the question always wants. The SI has asked only two of these in eight years, and both were answered by the first two sentences above rather than the formula. Learn the rates; the formula then follows.

The angle between the hands at 4:20
At 4:20 the minute hand is exactly on 4, but the hour hand has crept a third of the way past it. The gap is only 10°, not zero.
Both clock questions, solved (2)Tap to open
CHECK YOURSELF

A clock starts at midnight. By 6 : 10 a.m. the hour hand turns through _______________. (SI 18 Aug 2021)

  1. 1190 degrees
  2. 2185 degrees
  3. 3195 degrees
  4. 4200 degrees
Show answer

Answer: 2 — 185 degrees

The hour hand sweeps 360° in 12 hours, i.e. 30° per hour or 0.5° per minute. From midnight to 6:10 a.m. is 6 hours 10 minutes = 370 minutes. Angle turned = 370 × 0.5 = 185°. Equivalently 6 × 30 = 180° for the six full hours, plus 10 × 0.5 = 5° for the extra ten minutes. Option 1 (190 degrees) is the trap of crediting the hour hand with 1° per minute, which is the minute hand's rate divided by six - the minute hand itself turns 6° per minute.

CHECK YOURSELF

If the minute hand of a clock is pointing towards the north direction, then after 45 minutes, in which direction will the minute hand point? (SI 1 Jul 2023)

  1. 1North
  2. 2South
  3. 3East
  4. 4West
Show answer

Answer: 4 — West

The minute hand sweeps a full 360° in 60 minutes, so it turns 6° per minute, clockwise. In 45 minutes it turns 45 × 6° = 270° clockwise. Starting at North and turning clockwise: 90° → East, 180° → South, 270° → West. Option 3 (East) is what you get by turning anticlockwise, or by taking 90° instead of 270°.

COMMON TRAPS
  • Treating 1900 or 2100 as leap years. A century year is a leap year only if it divides by 400, so 2000 is and 1900 is not.
  • Forgetting that the hour hand moves between the hours. At 4:20 the hands are 10° apart, not together.
  • Counting the target year's own odd days. When moving from year A to year B, count the years from A up to but not including B.
  • Misreading “the day before yesterday”. Write today, yesterday and the day before in a column before answering.
  • Quoting “a calendar repeats after 11 years” as a rule. It is true only sometimes. Count the odd days instead.
QUICK BYTES
  • Ordinary year: 1 odd day. Leap year: 2.
  • Leap year: divisible by 4, century by 400.
  • 31-day month: 3 odd days. 30-day: 2.
  • Ordinary February: 0 odd days.
  • Jump whole years first, then days within the year.
  • A calendar repeats only for the same kind of year.
  • …and when the odd days between total a multiple of 7.
  • Minute hand 6° a minute; hour hand 0.5°.
  • Angle = | 30H − 5.5M |.
  • Over 180°? Subtract from 360°.

Frequently Asked Questions

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