A mixture contains milk and water in the ratio 3:1. On adding 5 litres of water, the ratio of milk to water become 2:1. The quantity of milk in the ratio is ?

Old ratio 30:10
New one 30:15

Let Milk be 3x litres
Water be x litres. As the ratio is 3:1

According to conditions 5L water is added,
So Water becomes x+5.

So 3x/x+5 = 2/1. x= 10.
We took milk as 3x= 3(10) = 30

3x / (x+5) = 2/1
on solving x = 10
So the milk qty is 3x = 3*10 = 30 litres