Apologia Physics: Math Exercise

During the first class, my husband, Rob, needs to get a feel for the kids' math aptitudes.  I use two tools: a math pretest similar to this one and a straight forward question, Should you buy a Prius to save money?  For this question, it makes sense to compare a Toyota Camry with a Camry Hybrid.  The hybrid is about $27000 and the Camry is about $22000.  (If you can obtain better figures, use them.  Be sure to compare similar models with the same options.). The posted mileage for the hybrid is 41 mpg and 28 mpg for the Camry.  Let's assume a person drives 15000 miles a year, does it pay to buy the Hybrid?  In our area, gas is about $2.00 per gallon.  Use your gas prices.
Camry:   15000/1 x 1gal/28 x $2.00/ gal = $1071.4
Hybrid:   15000/1 x 1gal/41 x $2.00/ gal = $731.7

The gas savings each year is $339.7; the cost difference between the two models is $5000.  So, you're saving $339.7 dollars each year.

$5000 / $339.7 = 14.7 years.

That means it takes almost 15 years in gas savings to meet the difference in price.  Now, this exercise doesn't take into consideration the difference in maintenance, the hybrid battery lifespan (which is reportably much better than it was), insurance, etc.  I do this exercise to see how the kids think while giving them something to think about the next time someone says they bought a hybrid to save money.