Repeated replacement — Aptitude Question
Repeated replacement: A tank holds 100 litres of milk. 20 litres is drawn off and replaced with water twice. Answer and worked solution on CodeKairo.
- Topic: Mixtures & Alligation
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 60 seconds
Question
A tank holds 100 litres of milk. 20 litres is drawn off and replaced with water twice. How much milk remains?
Options
- 60 L
- 64 L
- 68 L
- 72 L
Show the answer and the worked solution
Correct answer: B. 64 L
Worked solution
- Fraction remaining each time = 0.8.
- After two operations: 100 × 0.64 = 64 L.
Answer: 64 L.
The idea
Remaining = initial × (1 − drawn/total)ⁿ. Subtracting 40 litres directly would give 60 and ignores that the second draw removes less milk.
Shortcut. Each draw takes 20% of whatever milk is left: 100 → 80 → 80 − 16 = 64 L.
Common trap. "60 L" — subtracting 20 L of milk twice; the second draw is already diluted and removes only 16 L of milk.