Question 712426
Concentrate on the time interval.  You do not know how much water the pool holds; you do not know how much water is in the pool now; you do not know how much water was in the pool 5 hours ago.  You only know the flow rate of filling the pool and an interval of time toward the past.  


Five hours ago, {{{-23*5*60}}} gallons filled than now.
The reason for the factor 60 is because we want to convert the 5 hours into the equivalent minutes.  

The units are as so, (gallons/min)*(hours)*(minutes/hour)=gallons