Pipe P can fill a tank in 6 hours, pipe Q in 12 hours and pipe R in 24 hours. If all the pipes are open, in how many hours will the tank be filled? Pick: A) 2 hours B) 2.4 hours C) 3 hours D) 3.4 hours E) 4.5 hours

Rationale: "explanation : part filled by ( p + q + r ) in 1 hour = ( 1 / 6 + 1 / 12 + 1 / 24 ) = 7 / 24 all the three pipes together will fill the tank = 24 / 7 = 3.4 hours answer d"
Answer: D