two pipes filling a pool. how many hours does it take?

Question

A pool can be filled by one pipe in 3 hours and by a second pipe in 9 hours.  How long will it take using both pipes to fill the pool?

Answer 1

In one hour the fast pipe does 1/3 of the job.

In one hour the slow pipe does 1/9 of the job.

 

In x hours the fast pipe does x/3 of the job.

In x hours the slow pipe does x/9 of the job.

 

Working together they do the one job.

 

x/3 + x/9 = 1

3x + x = 9

4x = 9

x = 2.25

 

Together they work 2 and a quarter hours.