Three vessels contain a milk mixture 30 litre each. When put in a big vessel, they result in a mixture of milk and water in the ratio 2:1. If the ratio is to be reversed to make it 1: 2, how much more water should be added to the mix?

3 vessels of 30 litres each = 3 X 30 = 90 ml milk mixture
Amount of milk in mixture = 2/(2 + 1) × 90 = 60L
Amount of water = 90 - 60 = 30L

To make milk to water ratio 1 : 2, we simply need to make water double of milk.
By direct observation,
we can say that we should have 60L x 2 = 120L water to make the required ratio

We already have 30L,
we need (120 - 30) = 90L more water.