Answer : 4. "9:1"
Explanation :

Answer: D) 9:1 Explanation: Milk = 3/5 x 20 = 12 liters, water = 8 liters   If 10 liters of mixture are removed, amount of milk removed = 6 liters and amount of water removed = 4 liters.   Remaining milk = 12 - 6 = 6 liters Remaining water = 8 - 4 = 4 liters   10 liters of pure milk are added, therefore total milk = (6 + 10) = 16 liters.   The ratio of milk and water in the new mixture = 16:4 = 4:1   If the process is repeated one more time and 10 liters of the mixture are removed, then amount of milk removed = 4/5 x 10 = 8 liters.   Amount of water removed = 2 liters.   Remaining milk = (16 - 8) = 8 liters. Remaining water = (4 -2) = 2 liters.   Now 10 lts milk is added => total milk = 18 lts   The required ratio of milk and water in the final mixture obtained   = (8 + 10):2 = 18:2 = 9:1.