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
