Pipe P can fill a tank in 3 hours, pipe Q in 9 hours and pipe R in 24 hours. If all the pipes are open, in how many hours will the tank be filled?  Choose the correct choice
A) 2 hours
B) 2.1 hours
C) 3 hours
D) 3.5 hours
E) 4.5 hours

Rationale: "explanation : part filled by ( p + q + r ) in 1 hour = ( 1 / 3 + 1 / 9 + 1 / 24 ) = 35 / 72 all the three pipes together will fill the tank = 72 / 35 = 2.1 hours answer b"
Answer: B