Basically this question is telling you to convert units of distance and units of volume. The SI unit of measurement for distance is meters, while the SI unit for volume is liters. In order to go through the conversion, you have to know the equivalence of miles to meters, and gallons to liters. One gallon is equal to 3.785 liters, and one mile is equal to 1609.34 meters. Consequently, in the metric system, 1,000 meters is equal to 1 kilometer.
We make use of these conversions through dimensional analysis. This approach is when you cancel out like units if they appear both on the numerator and the denominator side. The solution is as follows:
40 miles/1 gal * (1609.34 m/1 mile) * (1 km/1000 m) * (1 gal/3.785 L)
Cancel out miles, gallons, and meters. The final answer is in kilometers per liter. The complete solution, is therefore, 17 km/L.