Two 4-litre replacements in 40 litres of milk — Aptitude
Two 4-litre replacements in 40 litres of milk: A vessel holds 40 litres of milk. 4 litres is removed and replaced… Answer and worked solution on CodeKairo.
- Topic: Mixtures & Alligation
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 75 seconds
Question
A vessel holds 40 litres of milk. 4 litres is removed and replaced with water, and this is done twice in all. How much milk remains?
Options
- 30.8 L
- 32.4 L
- 32.8 L
- 36 L
Show the answer and the worked solution
Correct answer: B. 32.4 L
Worked solution
- Each replacement removes one tenth of the milk, leaving 9/10.
- After two operations: 40 × (0.9)² = 40 × 0.81 = 32.4 L.
Answer: 32.4 L.
The idea
Repeated replacement: remaining = initial × (1 − drawn/total)ⁿ. It is compound decay with the same formula as depreciation.
Shortcut. Each draw takes 10% of whatever milk is there: 40 → 36 → 32.4.
Common trap. 36 L — stopping after one replacement; the second draw removes a further 3.6 L of milk.