A cistern is filled by pipe A in 20 hours and the full cistern can be leaked out by an exhaust pipe B in 25 hours. If both the pipes are opened, in what time the cistern is full?
Choose the correct choice from the following choices:  A) 50 hrs  B) 60 hrs  C) 70 hrs  D) 80 hrs  E) 100 hrs

Rationale: "time taken to full the cistern = ( 1 / 20 - 1 / 25 ) hrs = 1 / 100 = 100 hrs answer : e"
Answer: E