Every few days, on my way home from work, I pull into Ahmad's
Friendly Fuel and fill up my car's gas tank.
His price has been $2.29 per gallon for almost 4 years now, and it still is.
I don't always buy the same amount of gas, but no matter how much I buy,
my cost always fits the linear equation ...
Cost = (2.29) · (number of gallons) dollars.