A cistern is filled by pipe A in 10 hours and the full cistern can be leaked out by an exhaust pipe B in 12 hours. If both the pipes are opened, in what time the cistern is full?
Pick
A) 50 hrs
B) 60 hrs
C) 70 hrs
D) 80 hrs
E) 90 hrs

Rationale: "time taken to full the cistern = ( 1 / 10 - 1 / 12 ) hrs = 1 / 60 = 60 hrs answer : b"
Answer: B