Answer:
Sadie's family car is more fuel efficient.
Explanation:
Given;
Sadie's family car rating, S = 42 miles per gallon
Nanette's family car rating, N = 6 liters per 100 km
Sadie's family car rating, in liters per 100 km

Also, Nanette's family car rating, in liters per 100 km = 6 liters / 100km.
Thus, Sadie's family car uses lesser fuel, because of this Sadie's family car is more fuel efficient.