A car gets 30 miles per gallon of gas and is driven 1000 miles per month. Gas costs $\$4$ per gallon. How much will the car's owner save on gas expenses in one year by buying a new car that goes 50 miles for each gallon of gas?
Saving = old cost - new cost
Saving = ($4 * 1000/30 per month for a year) - ($4 * 1000/50 per month for a year)
Saving = (4000/30 * 12) - (4000/50 * 12)
Saving = (1600) - (960)
Saving = $640 a year