The answer is NOT 50cm.

Consider a rectangular open bath tub with a (constant) bottom surface area of A. With the tap open and the drain closed, it takes 20min to fill the bath to a height of 50cm. If we then close the tap and open the drain, the bath empties in 32min. The cross sectional area of the drain is B with BA and the drain is open to the atmosphere. What is the final water level if we open the tap with the drain open as well? Hint to solve this problem, analyze the three possible scenarios: A) Tap open, drain closed B) Tap closed, drain open C) Tap open, drain open