How much pure acid must be added to 20 litres of a 25% acid solution to make it 40% acid?

Explanation

1. 01

   Acid now: $0.25 \times 20 = 5$ litres. Adding $a$ litres of pure acid: $5 + a = 0.4(20 + a)$.

2. 02

   $5 + a = 8 + 0.4a$, so $0.6a = 3$ and $a = 5$ litres.