Diluting a salt solution — Percentages Aptitude Question
Diluting a salt solution: 80 kg of a solution contains 15% salt by weight. How much water must be added so that the… Answer and worked solution on CodeKairo.
- Topic: Percentages
- Section: Quantitative Aptitude
- Difficulty: Medium
- Time target: 75 seconds
Question
80 kg of a solution contains 15% salt by weight. How much water must be added so that the salt content falls to 10%?
Options
- 20 kg
- 30 kg
- 40 kg
- 50 kg
Show the answer and the worked solution
Correct answer: C. 40 kg
Worked solution
- Salt = 15% of 80 = 12 kg, and this stays fixed.
- For 12 kg to be 10% of the mixture, the new total = 12 ÷ 0.10 = 120 kg.
- Water to add = 120 − 80 = 40 kg.
Answer: 40 kg.
The idea
In dilution the solute is constant. Divide it by the target concentration to get the new total, then subtract the old total.
Shortcut. Strength × total stays constant: strength falls from 15 to 10 (× 2/3), so the total rises × 3/2, from 80 to 120 kg — 40 kg of water.
Common trap. "30 kg" — cutting the strength by a third tempts adding about a third of 80 kg (≈ 26.7) and taking the nearest option; the total must grow by half.