A cistern is normally filled in 2 hrs,but takes 2 hrs longer to fill because of a leak on its bottom,if Cistern is full ,how much time citern would empty? Choose one:
A) 4 hours B) 20 hours C) 30 hours D) 40 hours E) 50 hours

Rationale: "if leakage / hour = 1 / x , then 1 / 2 - 1 / x = 1 / 4 , solving 1 / x = 1 / 4 so in 4 hours full cistern will be empty . answer : a"
Answer: A