How many litres of water should be added to a 30 litre mixture of milk and water containing milk and...........

Explanation:

30 litres of the mixture has milk and water in the ratio 7 : 3.

i.e. the solution has 21 litres of milk and 9 litres of water.

When you add more water, the amount of milk in the mixture remains constant at 21 litres.

In the first case, before addition of further water, 21 litres of milk accounts for 70% by volume.

After water is added, the new mixture contains 60% milk and 40% water.

Therefore, the 21 litres of milk accounts for 60% by volume.

Hence, 100% volume =210.6=35 litres.

We started with 30 litres and ended up with 35 litres.

Therefore, 5 litres of water was added.

Hence, option 1 is the correct answer.