In a mixture of 40 litres, the ratio of milk and water is 4:1. How much water must be added to this mixture so that the ratio of milk and water becomes 2:3 ?

In a mixture of 40 litres, the ratio of milk and water is 4:1.

It means in the mixture there is $40\times \left(\frac{4}{5}\right)=32$ litres of milk and the remaining 8 litres of water

To make the mixture $2:3$ ratio of milk and water, required water is $3\left(16\right)=48$ litres

hence, the water must add in the mixture is 40 litres