A cistern which could be filled in 8 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 answer:
A) 76 hrs B) 99 hrs C) 55 hrs D) 90 hrs E) 40 hrs

Rationale: "1 / 8 - 1 / x = 1 / 10 = > 40 hrs answer : e"
Answer: E