Fifty litres of a mixture of milk and water contains 30% water. This mixture is added to eighty litres of another mixture of milk and water that contains 20% water. Then, how many litres of water should be added to the resulting mixture to obtain a final mixture that contains 25% water?

Fifty litres of a mixture of milk and water contains 30% water. This means that in this mixture, there are 15 litres of water and 35 litres of milk.

Eighty litres of another mixture of milk and water that contains 20% water. This means that in this mixture, there are 16 litres of water and 64 litres of milk.

Now, both mixtures are combined. There, the total quantity of the mix is 130 litres, of which 31 litres is water and 89 litres is milk. 

Now, we need to find out how many litres of water should be added to the resulting mixture to obtain a final mixture that contains 25% water.

=> $$\dfrac{31+x}{130+x}=\dfrac{1}{4}$$

=> $$124+4x=130+x$$

=> $$3x=6$$

=> $$x=2$$ litres of water should be added to make the concentration of water 25%.