I'm a big believer in financial literacy and the need to teach young people about saving money, building good credit and borrowing wisely.

But I'm not sure I helped the cause very much, at least not the part about saving money, when I showed a group of seventh- and eighth-grade students how to calculate the number of years it would take to double their money at current bank interest rates.

Here's how that went:

Using a trick called the rule of 70, you can figure out roughly how many years it will take to double your money through the magic of compounding interest. For example, if you earn 10 percent interest, your money doubles in roughly seven years. You learn this by dividing 70 by the rate of interest.

So if your money is earning 1 percent interest, it would take 70 years to double your money.

OK, kids, who can tell me how many years it takes to double your money at the rate many banks are offering on savings accounts, just one-tenth of 1 percent interest?

(In the classroom, eyes widened and jaws dropped as the middle schoolers quickly reached the correct answer). That's right, 700 years to double your money.

Ouch.

But here's the part that warmed my frugal heart. One of the kids in the class said she had opened a saving account at a bank that offered a cash promotion for new customers. She received a $50 bonus. Smart girl.

Finding deals

With interest rates so low that they are hardly worth mentioning, even enterprising middle school students are on the lookout for ways to wring out a few more dollars by taking advantage of promotional deals and incentives. I find some of the best offers in the circulars from grocery and drugstores.

For example, a week ago there was a coupon in the local Publix grocery store circular (it was in the May 4 editions of The Post and Courier and expired Wednesday) that allowed shoppers to buy a $50 gas card for $40, so long as they also spent $25 at the grocery store.

Had I noticed that one earlier, I would have bought some extra papers, hit the Publix several times and bought a bunch of gift cards.

Hey, a $50 gas card for $40 is a 25 percent return on your money, and I'm pretty confident my need to buy gas won't change soon. And it's not hard to meet a $25 purchase requirement at a grocery store -- that's a big bag of dog food and some laundry detergent.

Deals like that don't come along every day. But they do come along regularly. Prior to the gas card deal, Publix was offering a $50 American Express gift card for $40. Free money, even after paying the $4.95 fee to activate the card.

Buying discounted gift cards certainly won't replace earning a decent rate of return on your savings, but every little bit helps.

Visit postandcourier.com and see my November column, "Save money when buying gift cards," for related tips.

For more money-saving tips, go to postandcourier.com/personal_ finance.