So what you need to do is take 15 gallons and convert it into liters by multiplying by a ratio, which is given (1 gallon = 3.79 liters; 1g : 3.79L). The way you do this is by making sure the equation has a ratio that has the gallons on the bottom so that when you calculate how many liters there are, the gallons cancel each other out, leaving you with liters.
As shown in the picture:
