Answer
$\dfrac{1}{45}$ gallons of gasoline per mile driven.
This is around $0.022$ gallons of gasoline per mile driven.
Work Step by Step
After driving 90 miles, the car has consumed 2 gallons of gasoline.
After driving 225 miles, the car has consumed 5 gallons of gasoline.
The increase in gasoline consumed is:
$=5-2=3$ gallons
The increase in distance driven is
$=225 - 90=135$ miles driven
Thus, the average rate is:
$=\dfrac{3\text{ gallons of gasoline}}{135 \text{ miles driven}}=\frac{1}{45}$ gallons of gasoline per mile driven