A cistern is filled by pipe A in 16 hours and the full cistern can be leaked out by an exhaust pipe B in 20 hours. If both the pipes are opened, in what time the cistern is full? Pick one.
A) 50 hrs
B) 60 hrs
C) 70 hrs
D) 80 hrs
E) 90 hrs

Rationale: "time taken to full the cistern = ( 1 / 16 - 1 / 20 ) hrs = 1 / 80 = 80 hrs answer : d"
Answer: D