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?  Select the correct option
A) 20 hours
B) 15 hours
C) 10 hours
D) 12 hours
E) 7 4 / 9 hours

Rationale: "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 e"
Answer: E