SOLUTION: A 5 horsepower pump can empty a pool in 5 hours. A smaller 2 horsepower pump empties the same pool in 8 hours. The pumps are used together to begin emptying this pool. After 2 hour

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Let's break this down by work:
Let = the work that the 5 hp pump does each hour (If it can do the whole job in 5 hours, it is doing of the job each hour
Let = the work that the 2 hp pump does each hour.
Working together they do
= or of the job each hour
So in 2 hours, they have completed
This means that there is only of the job left to do.
we know that the rate of work * time = amount of work completed
so:
multiply both sides by 5:
or 1 and 3/4 hours.