If two water pipes can fill a swimming pool in 18 hours when both are open. One of the pipes can fill the pool alone in 45 hours , how long would it take the other pipe to fill the pool alone?
Ans:
Pipe A fills 1/45 of the pool per hour.
Let pipe B take x hours to fill the pool. So in one hour, it fills 1/x of the pool.
So in one hour, they together fill 1/x + 1/45 =  of the pool.
In 18 hours, they fill 100% of the pool. So the  equation is:

or

or
 or 

So the pipe B would take 30 hours to fill the pool by itself.

Hope this helps.