Reversing a milk-and-water ratio — Aptitude Question
Reversing a milk-and-water ratio: A 60-litre mixture has milk and water in the ratio 2 : 1. How much water must be… Answer and worked solution on CodeKairo.
- Topic: Ratio & Proportion
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 75 seconds
Question
A 60-litre mixture has milk and water in the ratio 2 : 1. How much water must be added to make the ratio 1 : 2?
Options
- 20 L
- 40 L
- 50 L
- 60 L
Show the answer and the worked solution
Correct answer: D. 60 L
Worked solution
- Milk = (2/3) × 60 = 40 L; water = 20 L.
- For milk : water = 1 : 2, water must be 2 × 40 = 80 L.
- Water to add = 80 − 20 = 60 L.
Answer: 60 L.
The idea
Anchor on the quantity that stays fixed, work out what the other must become, and subtract what is already there.
Shortcut. With the milk fixed, the water must go from half the milk to twice the milk — four times its present 20 L — so add 3 × 20 = 60 L.
Common trap. "20 L" — making the water equal the old 40 L of milk, which only reaches 40 : 40 = 1 : 1, not 1 : 2.