A person buys milk of rs. 8.50 per litre and add water to it. He sells the mixture at the same price and earns a profit of 11.11%. Find out how much water did he add to each litre of milk

We know that 1/9 = 0.1111

Cost price + profit = selling price

Lets assume the cost price of milk is x, the calculation would be:

x + 1/9 x = 1

10/9 x = 1

x = 9/10

The amount of water would be : 1 - 9/10 = 1/10 = 10%