anirban16 wrote:

In 40 liters of milk and water the ratio of milk and water is 3:1. How much water should be added in the mixture so that the ratio of milk to water becomes 2:1

(a) 6

(b) 5

(c) 3

(d) 8

(e) 4

With a 3:1 ratio in 40 liters, imply 30 liters of milk and 10 liters of water.

Using the options

(a) Lets add "6 liters" of water. With 30 liters of milk and (10+6) 16 liters of water, the ratio is 30:16 = 15:8. So this answer is wrong

(b) Lets add "5 liters" of water. With 30 liters of milk and (10+5) 15 liters of water, the ratio is 30:15 = 2:1.... Bingo... here is our answer.

No need to check any other answers, since in GMAT only one answer is true.

Ketan