How much water must be added to 30 litres of an 80% alcohol solution to make it 60% alcohol?

Explanation

1. 01

   Alcohol stays at $0.8 \times 30 = 24$ litres; it must be 60% of the new total.

2. 02

   New total $= \frac{24}{0.6} = 40$ litres, so add $40 - 30 = 10$ litres.