so, there are 3.785 liters to a US gallon, and there are 1.609 km to 1 mile
so, to convert 30 m/g to l/km [tex]\bf \cfrac{30mi}{1gallon}\cdot \cfrac{1gallon}{3.785L}\cdot \cfrac{1.609km}{1mi}\implies \cfrac{\boxed{?}}{\boxed{?}}[/tex]
simplify away, cancel units as needed