well, for a)
the car does 25mpg, how about for 100 miles?
well, 100 is 4 * 25, so that means for 100 miles, the car will use up 4 gallons
now, each gallon is $2.5, so the total cost for that will be 4 * 2.5
for b)
m = miles
g = gallons
d = distance in miles
[tex]\bf \begin{array}{llllll}
C=&d&\cdot &\cfrac{2.5}{g}&\cdot &\cfrac{25g}{m}\\
&\uparrow &&\uparrow &&\uparrow \\
&miles&&\textit{money per gallon}&&\textit{rate of consumption}\\
&&&&&\textit{of the car}
\end{array}[/tex]