A milkman mixed water with $60\text{litres}$ of pure milk and sold the mixture at the cost price, thereby making a profit of $25\%$. Find the quantity of water in the mixture.

Let the cost price of 1 litre of pure milk be $\text{Rs. }1$.
Cost Price of $60\text{litres}$ of pure milk = $\text{Rs. }60$.

Since the milkman sells the entire mixture at the cost price of pure milk, his entire profit comes from the volume of water added (since water is assumed to be free of cost).

$$\text{Profit Percentage}=\left(\frac{\text{Volume of Water}}{\text{Volume of Pure Milk}}\right)\times 100$$

$$25=\left(\frac{\text{Volume of Water}}{60}\right)\times 100$$

$$\text{Volume of Water}=\frac{25\times 60}{100}=15\text{litres}$$

Thus, the quantity of water added is $15\text{litres}$.