How much water should be added to 60 liters of milk at 1½ liters a rupee so as to have a mixture worth 1 7/8 liters a rupee?

Price of milk = 1/1.5 = 2/3 rupees per liter. Price of mixture = 1/(15/8) = 8/15 rupees per liter. Water costs 0. By alligation: ratio of water to milk = (2/3 – 8/15)/(8/15 – 0) = (2/15)/(8/15) = 1:4. For 60 liters milk, water = 60/4 = 15 liters. The answer is (d) 15 liters.