2017 AMC 10B Problem 12

Let the old car’s fuel efficiency be $M$ kilometers per liter and let gasoline cost $C$ dollars per liter. The old car therefore costs $\dfrac CM$ dollars per kilometer to fuel. The new car gets $1.5M$ kilometers per liter and its fuel costs $1.2C$ dollars per liter, so its fuel cost per kilometer is $$\dfrac{1.2C}{1.5M}=0.8\dfrac CM.$$ The new fuel cost is $80\%$ of the old one, so Elmer saves $20\%.$ Thus, the correct answer is A .