You may have already heard of the "Rule of 72". It seems easy enough: To figure out how many years it will take an investment to double its value, just divide 72 by the interest rate.

So if you're earning 6% APY, it'll take 72 / 6 = 12 years to double. If an investment has a 8% yield, you're looking at 72 / 8 = 9 years.

OK, so what's the problem? This seems straight-forward; why am I even writing a blog post about this?

Here's the thing: The Rule...