A cistern which could be filled in 4 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? Pick: A) 76 hrs B) 99 hrs C) 55 hrs D) 20 / 3 hrs E) 11 hrs

Rationale: "1 / 4 - 1 / x = 1 / 10 = > 20 / 3 hrs answer : d"
Answer: D