Equations & Word Problems

23 SI questionsAlgebraFree during launch
Maths: all chapters

Algebra in the Punjab Police SI paper is not about theory. Every one of the 32 algebra questions asked since 2016 is a sentence you have to turn into an equation, or an ugly expression that collapses the moment you spot the identity hiding in it. That gives you only two skills to build. Translate: give the unknown a letter, write one equation per sentence, solve. Recognise: learn eight identities so well that (0.93³ − 0.7³)/(0.93² + 0.651 + 0.7²) is a two-second answer instead of a calculator job. This note takes both, part by part — identities, linear pairs, quadratics, ages, variation and the single logarithm question — with every SI question solved.

32
SI questions
2016–2023
18
linear and quadratic
the biggest slice
9
on identities
a³ ± b³ leads
4
on ages
one table solves all

The eight identities

Nine of the 32 questions are one of these in disguise.

A grid of the eight standard algebraic identities, with the two cube identities highlighted
Learn these by shape, not by letters. Whatever sits in the a-slot and the b-slot — a number, a decimal, a cube root — the identity still holds.
KEY TERM
a³ + b³ = (a + b)(a² − ab + b²)

This single line, read backwards, is the SI examiner's favourite trick. When you see a fraction whose denominator is a² − ab + b² and whose numerator is a³ + b³, the whole thing is just 1/(a + b). The sign rule is worth saying aloud: same sign outside, opposite sign in the middle. a³ + b³ takes (a + b) outside and −ab in the middle; a³ − b³ takes (a − b) outside and +ab in the middle.

a³ + b³→(a + b)(a² − ab + b²)→divide → 1/(a + b)
QUICK CHECK
Simplify the following : ((7)^{1/3} + (3)^{1/3})((7)^{2/3} − (21)^{1/3} + (3)^{2/3}) (SI 2 Jul 2023)
Tap an option to answer
QUICK CHECK
Simplify : (6.28 × 6.28 − 3.4 × 6.28 + 3.4 × 3.4) / (6.28 × 6.28 × 6.28 + 3.4 × 3.4 × 3.4) (SI 2 Jul 2023)
Tap an option to answer
QUICK CHECK
(0.93³ − 0.7³)/(0.93² + 0.651 + 0.7²) = ? (SI 20 Jun 2023)
Tap an option to answer
THE TWO SQUARES YOU WILL USE MOST

a² + b² = (a + b)² − 2ab and a² + b² = (a − b)² + 2ab. A question almost never gives you a and b separately; it gives you their sum and their product, or their difference and their product. These two lines convert what you were given into what was asked, with no solving at all. The cube version works the same way: a³ − b³ = (a − b)³ + 3ab(a − b).

QUICK CHECK
If x = 2 + √5 and y = 2 − √5, then the value of x² + y² is : (SI 2 Jul 2023)
Tap an option to answer
QUICK CHECK
If 3x − 2y = 10 and xy = 11, then the value of 27x³ − 8y³ is _______. (SI 2 Jul 2023)
Tap an option to answer
QUICK CHECK
If a = 0.25 and b = 0.75, then the value of the expression 0.5/(b² − a²) is ______. (SI 29 Jul 2023)
Tap an option to answer
QUICK CHECK
What is the remainder when (x^{7} + 1) n is divided by (x + 1)? (SI 14 Jul 2023)
Tap an option to answer
QUICK CHECK
Simplify the expression given below. (x² + 3x − 54)/(x² − 36) + 9/(x + 6)² − 3/4 (SI 20 Jun 2023)
Tap an option to answer
QUICK CHECK
Simplify the following expression. (a^5 − a³b² − a²b³ + b^5)/(a^5 − 3a⁴b + 2a³b² + 2a²b³ − 3ab⁴ + b^5) (SI 29 Jun 2023)
Tap an option to answer

Turning sentences into equations

The largest group: fourteen questions.

Two linear equations in pens and pencils being scaled and subtracted to eliminate one unknown
Multiply each equation so that one unknown has the same coefficient in both, then subtract. Read the last line twice: the paper often asks for a combination, not for a single unknown.
FOUR STEPS THAT NEVER CHANGE
  1. Name the unknown
  2. One sentence, one equation
  3. Eliminate
  4. Answer what was asked
QUICK CHECK
The cost of 5 pens and 2 pencils is ₹29 and the cost of 7 pens and 5 pencils is ₹45. Find the cost of 1 pen and 2 pencils. (SI 16 Oct 2022)
Tap an option to answer
QUICK CHECK
In a group of cows and hens, the number of legs are 14 more than twice the number of heads. The number of cows is (SI 2016)
Tap an option to answer
QUICK CHECK
Roy is attempting a question paper where he gets 3 marks for every correct answer and 1 mark is deducted for every wrong answer. After attempting 60 questions, Roy sees that his total marks is 100. Find the number of questions he answered incorrectly. (SI 16 Oct 2022)
Tap an option to answer
QUICK CHECK
The sum of two numbers is 100. The difference of their squares is 6000. Find the larger number. (SI 9 Jul 2023)
Tap an option to answer
QUICK CHECK
A water tank is 2/3 full. When 30 litres are drawn from it, it is just 1/3 full. Half the capacity of the tank (in litre) is : (SI 2 Jul 2023)
Tap an option to answer
QUICK CHECK
When 2 is added to the numerator of a fraction and 5 is added to its denominator, the fraction becomes 1/2. When 2 is subtracted from both the numerator and the denominator, it becomes 1/3. Find the fraction. (SI 14 Jul 2023)
Tap an option to answer
QUICK CHECK
The equation 19 (x + y) + 17 = 19 (− x + y) − 21 is true for x is equal to : (SI 18 Aug 2021)
Tap an option to answer
QUICK CHECK
The sum of two times a number and three times 52 is 342. What will be the sum of four times that number and two times 52? (SI 20 Jun 2023)
Tap an option to answer
QUICK CHECK
A bonus was distributed to 12 workers instead of 16 workers. If each worker received $443 more, then find the total amount. (SI 18 Aug 2021)
Tap an option to answer
QUICK CHECK
If the length of a rectangle is either (x + 3y) units or 13 units, and its breadth is either (3x + y) units or 7 units, then find the value of y. (SI 24 Aug 2021)
Tap an option to answer
CHECK YOURSELF

If (3/5) × (x ÷ 6/5 − 2 ÷ 3) 5 = 3y and y = 4(4 ÷ 2/5 × 2 − 2 ÷ 5/3) ÷ 4/5, then which of the following is a whole number? (SI 14 Jul 2023)

Show answer

Work out y first, strictly left to right inside the bracket. 4 ÷ 2/5 = 10, and 10 × 2 = 20. Next 2 ÷ 5/3 = 6/5 = 1.2, so the bracket is 20 − 1.2 = 18.8. Then 4 × 18.8 = 75.2, and 75.2 ÷ 4/5 = 75.2 × 5/4 = 94. So y = 94. Now x. The left side is (3/5) × (5x/6 − 2/3) × 5 = 3(5x/6 − 2/3), and this equals 3y = 282. So 5x/6 − 2/3 = 94, giving 5x/6 = 284/3 and x = 568/5 = 113.6. Test the options: x + (y + 3)/5 = 113.6 + 97/5 = 113.6 + 19.4 = 133, a whole number. The other three give 359.6, 132.8 and 132.6.

Quadratics without the formula

Sum and product beat substitution every time.

A quadratic equation with arrows to the sum of roots, the product of roots and the discriminant
For ax² + bx + c = 0 the roots add to −b/a and multiply to c/a. Most SI questions need only these two numbers, never the roots themselves.
QUICK CHECK
If q > 0, and the roots of x² − 17x + q = 0 are integers, then how many values of 'q' can be integers? (SI 19 Aug 2021)
Tap an option to answer
QUICK CHECK
Find the roots of the equation x² + 2x − 6 = 0. (SI 19 Aug 2021)
Tap an option to answer
WHAT THE DISCRIMINANT TELLS YOU

D = b² − 4ac. If D > 0 there are two distinct real roots; if D = 0 the two roots are equal; if D < 0 there is no real root. If D is also a perfect square and a, b, c are integers, the roots are rational — which is exactly the test to apply when a question says “the roots are integers”.

Ages

Four questions, one table.

A two-row table showing a son's and a man's ages in the past, present and future
Give the present ages letters, then add or subtract the same number from every entry in a column. The condition in the question is always one row read across.
QUICK CHECK
A man is four times as old as his son. After 6 years, the man's age will be three times that of his son. The sum of their present ages is : (SI 31 Jul 2023)
Tap an option to answer
QUICK CHECK
Baarish is 8 years older than Radha. In five years, the sum of their ages will be 82. What are their present ages? (SI 14 Jul 2023)
Tap an option to answer
QUICK CHECK
In 10 years, A will be twice as old as B was 10 years ago. If A is now 9 years older than B, the present age of B is ____. (SI 29 Jul 2023)
Tap an option to answer
QUICK CHECK
A man's age is four times the sum of the ages of his two daughters. After six years, his age will be three times the sum of his daughters' ages. Find the man's present age. (SI 15 Jul 2023)
Tap an option to answer
THE SIX-PERSON AGE PUZZLE, ASKED THREE TIMES

The same puzzle about X, A, B, C, D and E appears in three different SI papers, each time asking for a different person. Solve it once, in this order: X = √25 = 5 · C = 12X = 60 · A = 10% of C = 6 · B = X + 2.8 = 7.8 · D × A = 10B ⇒ D = 78/6 = 13 · E: D + 2 = 2E ⇒ E = 7.5 Always start from the one sentence that gives a bare number — here “X is the square root of 25” — and chain outwards. In the 15 July 2022 version B is defined as 30% more than A instead, which changes B but not C, so the answer to “What is C's age?” is still 60.

CHECK YOURSELF

There are six persons X, A, B, C, D and E. A's age is 10% of C's age. After 2 years, D's age will be 2 times E's age. Also, the product of D's age and A's age is 10 times B's age, and C's age is 12 times X's age. B's age is 2.8 years more than X's age, which is the square root of 25. What is A's age? (SI 20 Jun 2023)

Show answer

X = √25 = 5 and C = 12X = 60, so A = 10% of 60 = 6. The rest of the data (B = 7.8, D = 13) is not needed for this one.

CHECK YOURSELF

There are six persons X, A, B, C, D and E. A's age is 10% of C's age. After 2 years, D's age will be 2 times E's age. Also, the product of D's age and A's age is 10 times B's age, and C's age is 12 times X's age. B's age is 2.8 years more than X's age, which is the square root of 25. What is D's age? (SI 29 Jul 2023)

Show answer

Same chain: X = 5, C = 60, A = 6, B = X + 2.8 = 7.8. Then D × A = 10B gives 6D = 78, so D = 13.

CHECK YOURSELF

There are six people X, A, B, C, D, E. B’s age is 30% more than A’s age, which is 10% of C’s age. D’s age 2 years later will be 2 times the age of E. Also, the product of D’s age and A’s age is 10 times that B’s age and C’s age is 12 times that of X’s age. E’s age is 1.5 times that of X’s age, which is the square root of 25. What is C’s age? (SI 15 Jul 2023)

Show answer

This version defines B as 30% more than A, but the question only asks for C. X = √25 = 5 and C = 12X = 60 — one line, and the rest of the puzzle can be ignored. Read what is asked before you solve anything.

Variation and the one logarithm question

Small, but they come back.

KEY TERM
p varies inversely as q² means pq² = k

Inverse variation is a constant product, not a constant ratio. Find k from the pair you are given, then put the new value in. Direct variation is the mirror image: y = kx, a constant ratio. The SI paper has asked the inverse-square version twice with different numbers, so learn the two-step drill — find k, then substitute — rather than the answers.

p ∝ 1/q²→pq² = k→k from the given pair
QUICK CHECK
Given that q = 8 for p = 2, p varies inversely as the square of q. Find the value of p when q = 16. (SI 29 Jul 2023)
Tap an option to answer
CHECK YOURSELF

Given that q = 4 for p = 1 and p varies inversely as square of q. Which of the following options is equal to the value of p for q = 8? (SI 31 Jul 2023)

Show answer

Same drill: k = pq² = 1 × 16 = 16. At q = 8, p = 16 ÷ 64 = 1/4. Again q doubled and p fell to a quarter.

QUICK CHECK
If x² + y² = z², then 1/(log_{(z+x)} y) + 1/(log_{(z−x)} y) = ? (SI 29 Jun 2023)
Tap an option to answer
COMMON TRAPS
  • Answering x when the question asked for x + y, for a sum of ages, or for half the capacity. Underline the last line of the stem before you start.
  • Squaring surds or decimals one at a time. If you see a conjugate pair or a difference of squares, use the identity — it is always faster and always exact.
  • Forgetting that a loose decimal in a denominator is usually ab. 0.651 is 0.93 × 0.7.
  • In an ages question with two children, adding 6 to the total instead of 12. Each person ages separately.
  • Treating inverse variation as a ratio. pq² = k is a product; doubling q divides p by four, not by two.
QUICK BYTES
  • a³ + b³ = (a + b)(a² − ab + b²)
  • a³ − b³ = (a − b)(a² + ab + b²)
  • a² + b² = (a + b)² − 2ab
  • Roots: sum = −b/a, product = c/a
  • D = b² − 4ac decides how many real roots
  • Remainder of p(x) ÷ (x + 1) is p(−1)
  • Inverse variation: pqⁿ = k
  • 1/log_b a = log_a b
REMEMBER

Same outside, opposite inside. a³ + b³ keeps the plus outside, (a + b), and flips to −ab in the middle. a³ − b³ keeps the minus outside, (a − b), and flips to +ab. Say it once before the exam and you will never mix up the four cube terms.