Need to add `60Ltr of Water`

The total mix is 60Ltr, which has 2:1 ratio of Milk and water.

Thus,

It contains 40Ltr. Milk and 20Ltr. Water.

Now to make the ratio of 1:2, we need to add water which doubles the current contain of 40Ltr milk

So we need 80 Ltr. of water which shows 1:2. we have 20Ltrs already, so need to add 60Ltr. of water more.