Two pipes A and B can fill a tank in 10hour and 15 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?  Select:
A) 2 hrs	B) 6 hrs	C) 5 hrs	D) 4 hrs	E) 1 hr

Rationale: "part filled by a in 1 hour = 1 / 10 part filled by b in 1 hour = 1 / 15 part filled by a + b in 1 hour = 1 / 10 + 1 / 15 = 1 / 6 both the pipes will together will fill the tank in 6 hours . answer is b"
Answer: B