A 40-litre mixture of milk and water contains 10% water. How much water must be added so that water forms 20% of the new mixture?

The mixture starts with 4 litres of water and 36 litres of milk. Adding $x$ litres of water gives $(4+x)/(40+x)=1/5$, so $20+5x=40+x$ and $x=5$. Check: 9 litres of water in 45 litres is exactly 20%. (Answering 4 forgets that the total volume grows too.)