The ratio of milk and water in 85 kg of adulterated milk is 27 : 7. The amount of water which must be added to make the ratio 3 : 1 is?

Milk = 85 x (27/34) kg = 135/2 kg = 67.5 kg
Water = 85 x (7/34) kg = 35/2 kg = 17.5 kg
Let Amount of water to be added is x,
∵ 67.5 / (17.5 + x) = 3/1
⇒ 3 (17.5 + x) = 67.5
∴ x =5
∴ Water to be added = 5 kg.