Problem Solving

A certain car uses one gallon of gasoline every 30 miles when it travels on the highway, and one gallon of gasoline every 20 miles when it travels in the city. When a car travels 4 miles on the highway and 4 additional miles in the city, it uses what percent more gasoline than if it travels 8 miles on the highway?

A. 15%
B. 20%
C. 22.5%
D. 25%
E. 50%

The OA is D

Source: Economist GMAT

On the highway, the car use 1 gallon per 36 miles
In the city, it uses 1 gallon per 20 miles
No. of gallons used to travel 4 miles on the highway = 4/30 = 0.133
No. of gallons used to travel 4 miles in the city = 4/20 = 0.2
No. of gallons used to travel 8 miles on the highway = 8/30 = 0.267
The % difference from gallons used to travel 8 miles on the highway =
$$\frac{0.333-0.267}{0.267}\cdot\frac{100}{1}$$
$$=\frac{0.066}{0.267}\cdot\frac{100}{1}$$
$$=24.7\approx25\%$$
Answer = option D