A can full of a mixture of two liquids A and B in the ratio of 7 : 5. When 9 liters of mixture are drawn off from the can and replaced by the same quantity of liquid B, the ratio of A and B in the can becomes 7 : 9.  The capacity of the can is:

36 liters

Initial ratio of A and B = 7 : 5

After drawn, ratio of A and B = 7 : 5       ............(i)

After adding B, new Ratio of A and B = 7 : 9 ............(ii)

Here,

From (i) and (ii), we can see that,

Ratio of liquid B increase = 9 - 5 = 4R                  (Due to addition of 9 ltr)

Therefore, 4R = 9

Final quantity = 16R = 4 × 9 = 36 liters.            [from equation (ii)]

*The last total quantity and the initial total quantity will be same.*

Hence, Initial qty. = 36 ltr = Capacity of the can