Remark
It depends on where you live. In the United States the conversion factor is 3.78 L = 1 US gallon. Car lots in Canada still use the Imperial Gallon because it is larger and you get more miles to the gallon with a large gallon.
Solution
1 US gallon = 3.78 L
x US gallons = 15 L
1/x = 3.78/15 Cross multiply
3.78 x = 15 Divide by 3.78
x = 15/3.78
x = 3.968
Note: Since the question says approximately you have to round to something. I would try 4 US gallons.
An Imperial gallon is 4.5 Liters if you live in Canada or the United States.
