Two pipes A and B can fill a tank in 18 hours and 24 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?
Choose the correct choice from the following:
A) 20 hours B) 15 hours C) 10 hours D) 10 2 / 7 hours E) 8 hours

Rationale: "part filled by a in 1 hour = 1 / 18 part filled by b in 1 hour = 1 / 24 part filled by ( a + b ) in 1 hour = 1 / 18 + 1 / 24 = 7 / 72 both the pipes together fill the tank in 72 / 7 = 10 2 / 7 hours answer is d"
Answer: D