Mohan has 12 litres of hand sanitiser that is 75% alcohol. How much water should he add to bring it down to 60% alcohol?

Explanation

1. 01

   Alcohol = $0.75 \times 12 = 9$ litres, which stays fixed.

2. 02

   For 9 litres to be 60%, the total must be $\frac{9}{0.60} = 15$ litres.

3. 03

   Water to add = $15 - 12 = 3$ litres.