An electric pump can fill a tank in 10 hours. Because of a leak in the tank, it took 20 hours to fill the tank. If the tank is full, how much time will the leak take to empty it?  Answers.
A) 10 hours B) 12 hours C) 20 hours D) 5 hours E) 15 hours

Rationale: "work done by the leak in 1 hour = 1 / 10 - 1 / 20 = 1 / 20 the leak will empty the tank in 20 hours answer is c"
Answer: C