Pipe P can fill a tank in 2 hours, pipe Q in 4 hours and pipe R in 12 hours. If all the pipes are open, in how many hours will the tank be filled? Select the correct option: A) 2 hours B) 1.2 hours C) 3 hours D) 1.5 hours E) 2.5 hours

Rationale: "explanation : part filled by ( p + q + r ) in 1 hour = ( 1 / 2 + 1 / 4 + 1 / 12 ) = 5 / 6 all the three pipes together will fill the tank = 6 / 5 = 1.2 hours answer b"
Answer: B