Pipe A can fill a tank in 8 hours, pipe B in 4 hours and pipe C in 24 hours. If all the pipes are open, in how many hours will the tank be filled?

2.4 hr

3 hr

4 hr

4.2 hr

Explanation:

Part filled by pipe A in 1 hour =$\dfrac{1}{8}$

Part filled by pipe B in 1 hour =$\dfrac{1}{4}$

Part filled by pipe C in 1 hour =$\dfrac{1}{24}$

Part filled by pipes A,B,C together in 1 hour=$\dfrac{1}{8}+\dfrac{1}{4}+\dfrac{1}{24}=\dfrac{10}{24}$

i.e, pipes A,B,C together can fill the tank in $\dfrac{24}{10}$ hour =2.4 hour.