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:

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