From a container, 6 liters milk was drawn out and was replaced by water. Again 6 liters of mixture was drawn out and was replaced by the water. Thus the quantity of milk and water in the container after these two operations is 9:16. The quantity of mixture is:

• A. 25
• B. 16
• C. 31
• D. 15

Answer: (D)

Let quantity of mixture be x liters.   Suppose a container contains x units of liquid from which y units are taken out and replaced by Water. After operations , the quantity of pure liquid =  x 1 - y x n  units, Where n = no of operations .    So, Quantity of Milk =  x 1 - 6 x 2      Given that, Milk : Water = 9 : 16    => Milk : (Milk + Water) = 9 : (9+16)   => Milk : Mixture = 9 : 25     Therefore,  x 1 - 6 x 2 x = 9 25   => x = 15 liters