A milkman has 20 liters of milk. He mixes 5liters of water (which is freely available) in 20 liters of pure milk. If the cost of pure milk is Rs. 18 per liter, then the profit of the milkman when he sells all the mixture at cost price is:

25 %

When Mixture is sold at cost price, then the profit is always equals to the amount of extra liquid added.

Therefore,

Profit = 5 L × 18 = 90

C.P. = 20 L × 18  = 360

Profit percent = \(\frac{90}{360}\) × 100 = 25%