Pipe A can fill a tank in 6 hours. Due to a leak at the bottom, it takes 9 hours for the pipe A to fill the tank. In what time can the leak alone empty the full tank?

A. 16 hours
B. 15 hours
C. 18 hours
D. 17 hours
Explanation:
Let the leak can empty the full tank in x hours 1/6 – 1/x = 1/9
=> 1/x = 1/6 – 1/9 = (3 – 2)/18 = 1/18
=> x = 18.