Question: one pipe can fill a tank in 2 hours?
Answer: Pipe A alone takes 2hrs hence it fills 1/2 of the tank in 1 hour. Similarly Pipe B and C fills 1/4 and 1/8 of the tank in 1 hour respectively. So together they'll fill (1/2 + 1/4 +1/8) = 7/8 part of the tank in one hour. If x is the volume of tank then rate at which it'll fill is 7x/8 per hr.