Pipe P can fill a tank in 8 hours, pipe Q in 12 hours and pipe R in 20 hours. If all the pipes are open, in how many hours will the tank be filled?
Choose the correct choice from the following answers:
A) 2 hours	B) 2.4 hours	C) 3.8 hours	D) 3.5 hours	E) 4.5 hours

Rationale: explanation : part filled by ( p + q + r ) in 1 hour = ( 1 / 8 + 1 / 12 + 1 / 20 ) = 31 / 120 all the three pipes together will fill the tank = 120 / 31 = 3.8 hours answer c
Answer: C