There's an easy way to figure out how quickly something will double when it's growing exponentially. Just divide 70 by the percent increase, and you've got the doubling time.

For example, if you hear that the population of your town is growing by 2% per year, that means it will double in just 35 years! (70/2=35)

It works in reverse, too: divide 70 by the doubling time to find the growth rate. If you hear that U.S. population is due to double in 70 years, you know that it's increasing at 1% per year (70/70=1). Isn't it amazing that just a 1% annual increase can cause a doubling in 70 years?

Remember this "Rule of 70" whenever you hear that something is growing by some percentage, or that something is growing steadily. That means it's going to double -- and probably sooner than you think!