Two pipes A and B can fill a tank in 25 hours and 35 hours respectively. If both the pipes are opened simultaneously, how much time will be taken to fill the tank? Choices: A) 20 hours B) 14 6 / 11 hours C) 10 hours D) 12 hours E) 8 hours

Rationale: "part filled by a in 1 hour = 1 / 25 part filled by b in 1 hour = 1 / 30 part filled by ( a + b ) in 1 hour = 1 / 25 + 1 / 30 = 11 / 150 both the pipes together fill the tank in 150 / 11 = 14 6 / 11 hours answer is b"
Answer: B