One of the estimating tricks I use the most is the Rule of 72 ( http://en.wikipedia.org/wiki/Rule_of_72 http://en.wikipedia.org/wiki/Rule_of_72 ), which tells

One of the estimating tricks I use the most is the Rule of 72 ( http://en.wikipedia.org/wiki/Rule_of_72 http://en.wikipedia.org/wiki/Rule_of_72 ), which tells you how long it takes for a number to double (or halve) given a yearly interest percentage (or inflation). Just divide 72 by the interest rate and you get the number of years it takes for your money to double. E.g. at a 3.6% interest rate (above inflation!) it would take about 20 years for your investment to double.