John is trying to drain a swimming pool. he has two pumps, but he can only use one at a time. he knows that the time a pump takes to drain the pool varies inversely with the power in watts of the pump. his old pump is a 40 watt pump, and it can drain the pool in 5 hours. how long would the job take if he uses his new 100 watt pump?

What do you mean this sentence?
the time a pump takes to drain the pool varies inversely with the power in watts of the pump,
The meaning would be : if we have a power in watts greater the time to drag the pool will be less because the relation between power and time is inverse.  

40 watts5 hours) 
100 watts  x

because the relation between power and time is inverse, we multiply directly.
40 watts(5 hours)=100 watts x
x=(200 watts hours) / 100 watts=2 hours.

the time would be 2 hours.   (the new pump is faster)