A dishonest milkman plans to adulterate milk by adding water. If water is freely available, how many liters of water should he add to 5 liters of milk, so that, by selling the milk at cost price, he makes a profit of 20%?

Let the milkman mix ‘x’ liters of water in 5 liters of milk.
Let the cost price of milk be Rs M per liter
So, cost price of 5 liters of milk = 5M
So, cost price of (5+x) liters of the mixture = 5M
Selling price of (5+x) liters of milk = (5+x)M
=> Profit = ((5+x)M - 5M)/5M = 20/100 = 1/5
=> 5Mx = 5M
=> x = 1
So, the milkman should mix 1 liter of water in every 5 liters of milk to make a profit of 20% by selling the mixture at the cost price of milk