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?

Correct option is C

The milkman buys 20 liters of milk at Rs. 50 per liter, costing him Rs. 1000.
He adds (x) liters of water, which costs Rs. 20 per liter.
Now, we need to calculate the cost of the water:
Cost of water = x   × Rs. 20 
The total cost of the milk and water together is:
Total cost = Rs. 1000 + Rs. 20x 
He then sells the entire adulterated mix at Rs. 60 per liter. The total revenue from selling the milk-water mixture (20 + x liters) at Rs. 60 per liter is:
Revenue = (20 + x) × Rs. 60 
Given that his profit is Rs. 400, we set up the equation:
Revenue - Total cost = Rs. 400 
 (20 + x) × Rs. 60 - (Rs. 1000 + Rs. 20x) = Rs. 400
Expanding and simplifying:
1200 + 60x - 1000 - 20x = 400 
40x + 200 = 400 
40x = 200 
 x = 5 liters
Thus, the milkman added 5 liters of water to the milk. This calculation shows that option (c) is correct.