You can multiply the decimal fraction by the number of ounces in a gallon. 0.39 gallons is 49.92 ounces. Round it to 50 ounces. Now, convert that to quarts by dividing by 32 ounces per quart - in this case, since it's only one quart and change, subtract 32 from the number of ounces to get 9 gallons, 1 quart, 18 ounces.
Outside the US people use a strange measurement system called "metric". It's a bizarre concept that uses multiple of ten.
Google "convert gallons to liters" to find the number of liters in question, then work in liters. Most measuring cups and coolers are marked in liters and mililiters, so the actual measurement is a trivial task.
9.39 gallons = 35.55 liters.
After you've done that a few times you can begin thinking in liters and your problems are solved.