A solution of 60 litres of acid and water contains 65% acid. How much water must be added to get a solution of 52% acid ?

Given that solution of 60 litres of acid and water contains 65% acid. 

60 liters -- 65% acid

To get 52% acid, add some more water, so it is inverse proportion

hence, 

$\begin{array}{rcl}\frac{52x}{100}& =& \frac{65}{100}\left(60\right)\\ 52x& =& 65\left(60\right)\\ x& =& \frac{65\times 60}{52}\\ & =& 75\end{array}$ 

Hence number of liters of water must be added to get a solution $52\%$ acid is $75-60=\boxed{15}$