Divide 72 by your interest rate and you'll know how long your money takes to double
Want to know how many years it takes an investment to double at a given annual growth rate? Divide 72 by the rate: at 8% a year, money doubles in about 9 years; at 6%, about 12. The trick works because it approximates the natural logarithm of 2 divided by the growth rate — and 72 divides evenly by more common interest rates than the mathematically 'purer' number would.