Given the task definition and input, reply with output. The task is to generate the correct answer for a given math problem.

Problem: a milk man has 40 liters of milk . if he mixes 5 liters 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 :
45 %