In a 729 litres mixture of milk and water, the ratio of milk to water is 7:2. to get a new mixture containing milk and water in the ratio 7:3, the amount of water to be added is:

Explanation:-

Quantity of milk in 729 litre of mixture
= 7*729/9 = 567 litre
Quantity of water
= 729-567 = 162 litre.
Let x litre of water be added to become ratio 7:3.

Or, 7/3 = 567/(162+x)
Or, 162*7 +7x = 567*3
Or, 7x = 1701-1134 = 567
Or, x = 567/7 = 81 litre water is to be added.