50 litres of brine are 10% salt. How much water must evaporate to make it 25% salt?

Explanation

1. 01

   Salt stays at $0.1 \times 50 = 5$ litres; it must be 25% of what remains.

2. 02

   Remaining $= \frac{5}{0.25} = 20$ litres, so $50 - 20 = 30$ litres evaporate.