hi,

in 20 litre of mixture there is 60% milk, so the milk in the mixture is 20* 60% = 12 litre of milk present in the mixture.

now you have added 5 litre of pure milk in mixture. so new milk value is 12+5=17 litre.

but the new amount of mixture is 20+5= 25 litre.

so the new percentage of milk is (17/25)*100

so the new milk percentage is 68%.