33 questions from actual SSC CGL papers · all shown free · click option to reveal solution
RATIO & PROPORTION — CORE CONCEPT A ratio compares two quantities of the same kind. If you have 3 apples and 5 oranges, the ratio of apples to oranges is 3:5. That's it. A ratio is just a way of saying 'how many times one quantity is of another.'
A proportion says that two ratios are equal. So if 3:5 = 6:10, these four numbers are in proportion. ---
A ratio a:b is the same as the fraction a/b.
Both terms of a ratio can be multiplied or divided by the same number. The ratio stays equal. So 4:6 = 2:3.
A ratio has no units. You cannot take a ratio of 3 kg and 5 metres.
Order matters. 3:5 is NOT the same as 5:3.
In a proportion a:b = c:d, the outer terms (a and d) are called EXTREMES. The inner terms (b and c) are called MEANS.
Proportion Rule (Cross Multiplication):
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— WHAT GETS ASKED SSC CGL asks ratio problems in these forms: - Dividing a sum in a given ratio - Finding the original quantities when ratio changes after adding/subtracting a number - Age problems using ratio - Income/Expenditure ratio problems - Finding third or fourth proportional - Mixture problems using ratio The most common question type: 'A sum is divided in ratio a:b. One part is X. Find the other part or total.' --- EXAM SHORTCUTS AND TRICKS TRICK 1 — DIVIDING A SUM If a total amount T is divided in ratio a:b: First part = T x a/(a+b) Second part = T x b/(a+b) Example: Divide Rs 880 in ratio 3:5 First part = 880 x 3/8 = Rs 330 Second part = 880 x 5/8 = Rs 550 TRICK 2 — RATIO CHANGE TRICK If ratio of A and B is a:b and you add x to both, the new ratio becomes p:q. Then: A = x(p - ratio coefficient) — solve via cross multiplication directly.
Set A = ak and B = bk, form equation. TRICK 3 — UNIT VALUE METHOD (UNITARY METHOD) Find the value of one unit (1 part), then scale up. This is the fastest method for almost all basic ratio problems. ---
Add all ratio parts. 2 + 3 + 5 = 10 parts total.
Value of 1 part = 1200 / 10 = Rs 120.
A = 2 x 120 = Rs 240. B = 3 x 120 = Rs 360. C = 5 x 120 = Rs 600.
Check: 240 + 360 + 600 = 1200. Correct. --- WORKED EXAMPLE 2 Question: The ratio of A to B is 3:4. If 6 is added to each, the new ratio becomes 4:5. Find A and B.
Let A = 3k and B = 4k.
After adding 6: (3k + 6)/(4k + 6) = 4/5
Cross multiply: 5(3k + 6) = 4(4k + 6)
15k + 30 = 16k + 24
30 - 24 = 16k - 15k → k = 6
A = 3 x 6 = 18. B = 4 x 6 = 24. Verify: (18+6):(24+6) = 24:30 = 4:5. Correct. ---
— THE NUMBER ONE TRAP Students forget that when a ratio changes after adding or subtracting a value, they MUST set up the variable (let the ratio be 3k:4k) and form an equation. Many students directly assume k=1 and write A=3 and B=4. This is WRONG unless the question specifically states absolute values.
Always use 'k' as the multiplier. This single mistake causes maximum marks loss in ratio problems.
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