From a can full of milk, 4 litres are taken out and replaced with water, twice. Milk and water are then in the ratio 16 : 9. How much does the can hold?

Explanation

1. 01

   Milk is now $\frac{16}{25}$ of the can, so $\left(1 - \frac{4}{V}\right)^2 = \frac{16}{25}$.

2. 02

   $1 - \frac{4}{V} = \frac{4}{5}$, so $\frac{4}{V} = \frac{1}{5}$ and $V = 20$ litres.