Strengthening a spirit solution — Aptitude Question
Strengthening a spirit solution: How much pure spirit must be added to 60 litres of a 40% spirit solution to make… Answer and worked solution on CodeKairo.
- Topic: Mixtures & Alligation
- Section: Quantitative Aptitude
- Difficulty: Hard
- Time target: 105 seconds
Question
How much pure spirit must be added to 60 litres of a 40% spirit solution to make it 50% spirit?
Options
- 10 L
- 12 L
- 15 L
- 20 L
Show the answer and the worked solution
Correct answer: B. 12 L
Worked solution
- Water = 60% of 60 = 36 L, and it never changes.
- For water to be 50% of the final mixture, the total = 36 ÷ 0.5 = 72 L.
- Spirit added = 72 − 60 = 12 L.
Answer: 12 L.
The idea
Anchor on the component not being added. Tracking the water needs one division; tracking the spirit needs an equation.
Shortcut. Alligate pure spirit (100%) against the 40% solution at 50%: (50 − 40) : (100 − 50) = 1 : 5, so spirit = 60 ÷ 5 = 12 L.
Common trap. "10 L" — the most tempting wrong pick: reading the 10-point rise from 40% to 50% as 10 litres, when the added spirit also enlarges the total.