Two pipes A and B can fill a tank in 24 hours and 36 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?
Select:
A) 20 hours	B) 15 hours	C) 14 2 / 5 hours	D) 12 hours	E) 8 hours

Rationale: "part filled by a in 1 hour = 1 / 24 part filled by b in 1 hour = 1 / 36 part filled by ( a + b ) in 1 hour = 1 / 24 + 1 / 36 = 5 / 72 both the pipes together fill the tank in 72 / 5 = 14 2 / 5 hours answer is c"
Answer: C