Question

You drive a car that holds 15 gallons of gasoline. Your car uses 0.03 gal per mi. Which equation best represents this information?


A. y = 0.03x + 15
B. y = 0.03x - 15
C. y = -0.03x + 15
D. y = -0.03x - 15

Answer 1

y = -0.03x + 15

Answer 2

Since the tank holds 15 gallons, you must have positive 15 to represent the full tank.
Then for each mile you drive, you use 0.03 gal. That means you are using 0.03x gallons for x miles driven. Using gas is negative. It empties out the tanks, so the 0.03x part must be subtracted from the 15 gallons. That means y = 15 - 0.03x. If you write it with the x-term first, you get y = -0.03x + 15.