We have 20 litres of water and milk mixture
60%of it is milk.
Then the amount of milk in 20litres  is 20*.6=12 litres .
Now 5 litres of milk is added to it.
Hence the amount of mixture is 20+5=25 litres.
And the amount of milk in it is 12+5=17 litres .
Hence the percentage of milk in the mixture is 17*100/25=68%