Concept: The question tests on the area of Rates(Time and Work)
Pipe A Takes 2 hours to fill the tank alone.
Work done in 1 hour = 1/2
Let the Leak take x hours to empty the tank.
In 1 hour, work done = 1/x
Pipe A filling and the leak emptying take 2.5 hours to fill the tank.
Work done in 1 hour = 1/2.5 = 2/5
x = 10 hours