How much water must be added to 12 litres of a 60% alcohol solution to make it 40% alcohol?

Explanation

1. 01

   The alcohol stays $0.6 \times 12 = 7.2$ litres; it must be 40% of the new volume $V$: $0.4V = 7.2$, so $V = 18$.

2. 02

   Water to add $= 18 - 12 = 6$ litres.