A cistern is filled by pipe A in 10 hours and the full cistern can be leaked out by an exhaust pipe B in 15 hours. If both the pipes are opened, in what time the cistern is full?  Choose the correct choice from the following answers.
A) 50 hrs  B) 30 hrs  C) 70 hrs  D) 80 hrs  E) 90 hrs

Rationale: "time taken to full the cistern = ( 1 / 10 - 1 / 15 ) hrs = 1 / 30 = 30 hrs answer : b"
Answer: B