Two pipes A and B can fill a tank in 12 hours and 15 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank? Pick.
A) 20 hours	B) 15 hours	C) 10 hours	D) 12 hours	E) 7 4/9 hours
Part filled by A in 1 hour = 1/12   Part filled by B in 1 hour = 1/15   Part filled by (A+B) in 1 hour = 1/12 + 1/15 = 9/60   Both the pipes together fill the tank in 60/9 = 7 4/9 hours   Answer is 7 4/9 hours
<result>E</result>
Two pipes A and B can fill a tank in 12 hours and 15 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?