SSC CGL 2025 Tier-I · 14 September 2025, Shift 3 · Quantitative Aptitude · Question 11 of 95

We transcribed this text from the pictures on a candidate's response sheet for this shift; the answer is the key SSC colour-marked on that sheet. Solution last reviewed 13 September 2026.

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A solution consists acid and water in the ratio 5:3. If 4 litres of water is added, the new ratio becomes 5:4. What is the original quantity of acid in the solution?

(a)20 litres
(b)25 litres
(c)30 litres
(d)35 litres

Answer

Why

Correct — A.
Write the original solution as acid 5x, water 3x.
Add 4 litres of water: acid 5x, water 3x + 4.

Set the new ratio: 5x ÷ (3x + 4) = 5 ÷ 4
Cross-multiply: 20x = 15x + 20
Solve: 5x = 20, so x = 4

Original acid = 5x = 5 × 4 = 20 litres → option (a)

Why the others are wrong

(b)
25 litres — 25 litres of acid means 15 litres of water, and 15 + 4 = 19. The new ratio would be 25 : 19, not 5 : 4, which needs 20 litres of water.
(c)
30 litres — 30 litres of acid means 18 litres of water, then 18 + 4 = 22. That gives 30 : 22 = 15 : 11, not the 5 : 4 the stem states.
(d)
35 litres — 35 litres of acid means 21 litres of water, then 21 + 4 = 25. That gives 35 : 25 = 7 : 5, not 5 : 4.

Concept

When only one component is added, the other stays fixed and anchors the working. Here the acid is 5 parts in both ratios, so one part is the same size before and after.

The water went from 3 parts to 4 parts, and the one thing added was 4 litres of water. So 1 part = 4 litres, and the acid, 5 parts, is 20 litres.

Check: 20 litres of acid with 12 of water is 5 : 3. Adding 4 litres makes the water 16, and 20 : 16 = 5 : 4.

Key facts

If a ratio's unchanged term keeps the same number of parts, one part keeps the same size.
The original solution held 20 litres of acid and 12 litres of water, 32 litres in all.
Cross-multiplying a⁄b = c⁄d gives ad = bc.

Study next

Mixtures where the unchanged term has different part-numbers in the two ratios and must be rescaled firstRemoving part of a mixture and replacing it with one componentFinding the water to add for a target percentage of water

Common traps

Comparing parts across two ratios whose unchanged term differs: against a new 10 : 9, rewrite the old 5 : 3 as 10 : 6 first.
Adding the 4 litres to the acid term because acid is written first in the ratio.

17 Sep 2025, 12:30, Quant Q.10 is the same item with alcohol and water: 7 : 4 becomes 7 : 6 after 12 litres of water, so 2 parts = 12 litres and the barrel held 11 × 6 = 66 litres.

23 Sep 2024, 09:00, Quant Q.20 asks instead how much water to add: 20 litres of milk needs the water raised from 12 to 20 litres, so 8.

Related PYQs

No directly related past PYQ was found.

Topic: Ratio & Proportion — all 96 SSC CGL questions on it, every solved shift.