A milkman makes 20% profit by selling milk mixed with water at Rs.9 per liter. If the cost price of 1 liter pure milk is Rs.10, then the ratio of milk and water in the mixture is.

3 : 1

S.P. of mixture = Rs.9 per liter

Profit earn = 20% = +\(\frac{1}{5}\)

C.P. of mixture = 9 × \(\frac{5}{6}\) = Rs.7.5 

In Rs.7.5 the qty. of pure milk should be = 750ml

(because C.P. of 1000 ml pure milk = Rs.10)

Therefore, in 1000ml of mixture:-

Pure milk = 750ml

Water = 250ml

The ratio of milk and water = 750 : 250 = 3 : 1