A cistern which could be filled in 6 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?
Choose the correct choice from the following answers:	A) 76 hrs	B) 99 hrs	C) 15 hrs	D) 90 hrs	E) 11 hrs

Rationale: "1 / 6 - 1 / x = 1 / 10 = > 15 hrs answer : c"
Answer: C