Real-world math.

Has you kid ever said, "Why do I have to know math?" Answer: So we don't get ambushed!

Here is a real-life example:

Companies are trying really hard to promote themselves as environmentally friendly. I heard a "green" ad on the radio last week from one of the railroad companies. They said that they could haul 1 ton of freight 423 miles on a single gallon of gas. When I first heard that it sounded great!

So...I did some rough calculations:

If a single locomotive hauls 5000 tons of freight, how many miles per gallon does the locomotive really get?

5000 tons = 5000 gallons of gas for 423 miles (it's what they said on the ad)
423 miles divided by 5000 gallons = 0.085 miles per gallon
or
5000 gallons divided by 423 miles = 11.82 gallons per mile.

Granted, rail transportation is probably a lot more efficient than any other type of ground transportation. It just doesn't sound as "green" to say you ONLY use 11 gallons of gas for every mile you go. It's not exactly the next hybrid vehicle.

The ad ends by saying something like, "too bad we all can't drive locomotives." Yeah, then instead of using one gallon of gas to get to work I could use 201 gallons!