A cistern which could be filled in 9 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:  A) 76 hrs  B) 99 hrs  C) 55 hrs  D) 90 hrs  E) 11 hrs

Rationale: "1 / 9 - 1 / x = 1 / 10 = > 90 hrs answer : d"
Answer: D