I recently stumbled across an explanation of the Rule of 72, and it struck me as exactly the kind of thing I should know (but didn’t):
In finance, the Rule of 72 is a simple method of calculating the approximate number of periods over which a quantity will double. If you divide 72 by the expected growth rate, expressed as a percentage, the answer is approximately the number of periods to double the original quantity. For instance, if you were to invest $100 at 9% per annum, then your investment would be worth $200 after 8.0432 years, using an exact calculation. The rule of 72 gives 72/9=8 years, which is close to the exact answer.
I knew one instance of the rule: 10% interest will double your money after 7 years.