Each of three pumps A, B, C can pump water either into, or out of, a swimming pool. Pump A can fill the pool in 6 hours, while pumpp B works twice as fast. Pump C however takes one third more time than pump A to fill the pool.

Question

1. All three pumps together pumping in water will fill the pool in ____ hours.
2. With pump B pumping out water, and both pumps A and C pumping in, the pool wil empty in _____ hours.

Answer 1

1/6 + 1/3 + 1/(4/3*6) = 5/8
So, they will take 8/5 hr or 1 hr 36 min.

Now just figure

1/3 - (1/6 + 1/8) = 1/24
so it takes 24 hrs to empty the pool

Answer 2

The speed of a boat in still water is 20 mph. It travels 24 mi upstream and 24 mi downstream in a total time of 10 hr. what is the speed if the current?