A milkman buys 20 litres of milk from a dairy at the cost of Rs. 50 per litre. He also purchases some mineral water at the rate of Rs 20 per litre and adds it to the milk. He then sells the adulterated milk at the rate of Rs 60 per litre and makes a profit of Rs 400. What is the amount of water he added to the milk ?
1
10 litre
2
8 litre
3
5 litre
4
4 litre