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Q: What's the "Rule of 72"? - E.W., Mobile, Ala.

A: It offers a quick and easy way to figure out how long it will take for your money to double at various rates of growth. Imagine that your money is earning 4 percent in interest annually. Take 72 and divide it by 4 and you'll get 18, meaning that it will take roughly 18 years for you to double your money. If you're earning 10 percent interest (lucky you!), you'll double your money in about 7.2 years.

The results from this rule aren't precise, but they're pretty close for growth rates up to about 15 percent and aren't too far off even at 25 percent.

The rule also works in reverse.

If you want to double your moolah in six years, just divide 72 by 6 and you'll see that you'll need an average growth rate of roughly 12 percent.

Q: Can I buy fewer than 100 shares of stock in a company?

- J.L., Salisbury, Md.

A: You sure can. You can usually buy as little as one share at a time. Just pay attention to the commissions you pay your broker. If you buy one $50 share of stock and pay a $15 commission, you're out 30 percent from the get-go. It's sometimes best to accumulate cash and buy more at once, later.

If you're buying stock directly from a company, such as through a dividend reinvestment plan (a "DRIP"), your money can buy fractions of shares at a time. For example, a $50 contribution would buy you 0.67 shares of a $75 stock. Learn more about DRIPs at www. fool.com/School/DRIPs.htm and www. dripcentral.com and more about picking a good brokerage at www.broker. fool.com.

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