From the "Secrets of Millionaire Investors":
Albert Einstein, the greater genius of our time once remarked that compound interest was the greatest mathematical discovery ever made! He came up with a formula called the Rule of 72. It states that if you take 72 and divide it by the Annual Percentage return, it will give you the number of years your investment would double!
And, I did a few calculation to discover that Rule of 72 is bulletproof. Gosh, I admire his talent with numbers. ^^