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A solution of 100 litres has a 60% salt concentration. How many litres of water must be added to reduce the concentration to 40%, and then how much pure salt must be added to bring the concentration back to 50%?

1. Add 50 L of water, then 30 L of salt
2. Add 90 L of water, then 70 L of salt
3. Add 70 L of water, then 30 L of salt
4. Add 50 L of water, then 10 L of salt

Correct answer: Add 50 L of water, then 30 L of salt

Solution

Initially, salt = 60 L and water = 40 L. To make concentration 40%, the total must be \(60/0.4 = 150\) L, so 50 L water is added. Then to make concentration 50% with 60 L salt, total must be 120 L, so 30 L pure salt must be added.

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