Two pipes A and B can fill a tank in 10 hours and 20 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank?
Pick:
A) 10 / 3
B) 20 / 3
C) 15 / 7
D) 11 / 17
E) 1 / 4

Rationale: "part filled by a in 1 hour = 1 / 10 part filled by b in 1 hour = 1 / 20 part filled by ( a + b ) in 1 hour = 1 / 10 + 1 / 20 = 3 / 20 both the pipes together fill the tank in 20 / 3 hours answer is b"
Answer: B