If you will remember that the word "per" means "divided by" or "over",

then you need only look and observe that you are being asked for, that is,

"gallons per minute".  So translate that as "Gallons divided by minutes" or "Gallons over minutes".

So we have a 1-gallon drum filled in 3 minutes, so we put 1 gallon over 3 minutes:



Then separate the numbers from the words for the units:



That's "one-third of a gallon per minute".

Edwin