Strengthening an acid solution — Aptitude Question
Strengthening an acid solution: How much pure acid must be added to 50 litres of a 20% acid solution to make it 50%… Answer and worked solution on CodeKairo.
- Topic: Mixtures & Alligation
- Section: Quantitative Aptitude
- Difficulty: Hard
- Time target: 120 seconds
Question
How much pure acid must be added to 50 litres of a 20% acid solution to make it 50% acid?
Options
- 20 L
- 25 L
- 30 L
- 35 L
Show the answer and the worked solution
Correct answer: C. 30 L
Worked solution
- Acid = 20% of 50 = 10 L; water = 40 L, and the water never changes.
- In the final mixture water is 50%, so the total = 40 ÷ 0.5 = 80 L.
- Acid to add = 80 − 50 = 30 L.
Answer: 30 L.
The idea
Anchor on the component that is not being added — the water. Tracking the acid instead needs an equation, while tracking the water is a single division.
Shortcut. Alligate the 20% solution with 100% acid at 50%: (100 − 50) : (50 − 20) = 5 : 3, so 50 L of solution takes 30 L of acid.
Common trap. 25 L — adding half the volume on the hunch that half makes 50%; it gives 35 L of acid in 75 L, only about 46.7%.