Question

I am not exactly sure how to begin this problem.

Emptying a pool: A large pump can empty a swimming pool in 100 hours, whereas a small pump can empty a pool in 160 hours. how long will it take to empty the pool if both pumps are used?

Answer 1

Here is a place to start: The large pump empties 1/100 of the pool in an hour. So in x hours, it empties x(1/100) of the pool or x/100.
The small pump empties 1/160 of the pool in an hour. So in x hours, it empties x91/160) of the pool or x/160
Working together, the two pumps empty 1 pool.