so there are 2 things you need to convert, the first is the gallons to cups and the second is the hours to minutes. so I like to set it up with numerators and denominators and then multiply the ratios. All same terms will cross out and be opposite of each other.
(18 gal/1 hour) x (1 hour / 60 minutes), now we have (0.3 gallons/1 minute) we just have to change gallons to cups
(0.3 gallons/ 1 minute) x (4 quarts/1 gallon) x (2 pints/1 quart) x (2 cups/1 pint)
this comes out 4.8 cups/1 minute