There is a crack at the bottom of a tank. Before the crack appeared, Pipe A could fill the tank in 2 hours. Now it takes half an hour longer. How long will the crack take to empty a full tank when Pipe A is kept closed?

Correct Answer: 10 hours

Before the crack appeared, Pipe A could fill 1/2 the tank in an hour.

Now it takes 2 hours 30 minutes to fill the tank. That is 5/2 hours.

Hence, it fills 2/5th of the tank every hour.

The crack at the bottom accounts for this reduction in the amount.

The crack, therefore, drains 1/2−2/5=5−4/10=1/10th of the tank every hour.

As the crack drains a tenth of the tank every hour, it will drain the entire tank in 10 hours if Pipe A is kept closed.
Hence ,
10 hours