Describe a real-world situation that can be represented by a linear relationship?

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.