A tank gets filled by a pipe in 2 hours. But due to a leak, it takes 30 more minutes to fill the tank. How long will it take a full tank to get emptied by the leak, when the pipe is closed?

Correct option is A

Given:
Time to fill the tank = 2 hours, Extra time due to leak = 30 minutes = 0.5 hours.
Solution:
Combined work of pipe and leak: 1/2 - 1/t = 1/2.5
Without leak, pipe fills in 2 hours, so filling rate = 1/2.
With leak, it fills in 2.5 hours, so rate = 1/2.5.
Let the leak take time t to empty the tank. We have:
1/2 - 1/t = 1/2.5
=> 1/t = 1/2 - 1/2.5 = (5 - 4)/10 = 1/10
Therefore, t = 10 hours.
Alternate Method:

Total work = 10 units

Let efficiency of leak be x, then

5 – x = 4

=> x = 5 – 4

=> x = 1 units/hr

∴ Leak empty the tank in = 10/1 = 10 hrs