A chemist has 10 litres of a solution that is 10 per cent nitric acid by volume. He wants to dilute the solution to 4 per cent strength by adding water. How many litres of water must he add?

Out of 10 litre of solution, there is I litres of nitric acid and 9 litres of water. Let x litres of water be added to the solution so that the diluted solution is of 4% strength.

$\therefore 4\mathrm{\%}\text{ of }(10+x)=1\Rightarrow x=15\text{. }$