How many litres of water must be added to 24 litres of a 35% acid solution to make it a 21% acid solution?

Explanation

1. 01

   Water is a 0% acid solution.

2. 02

   Solution : water = $(21 - 0) : (35 - 21) = 21 : 14 = 3 : 2$.

3. 03

   3 parts = 24 litres, so water = $2 \times 8 = 16$ litres.