A cistern which could be filled in 5 hours takes one hour more to be filled owing to a leak in its bottom. If the cistern is full in what time will the leak empty it?  Select:
A) 76 hrs	B) 99 hrs	C) 55 hrs	D) 90 hrs	E) 10 hrs

Rationale: "1 / 5 - 1 / x = 1 / 10 = > 10 hrs answer : e"
Answer: E