The rule of $70$ applies in any growth rate application. Let's say you have $$1,000$ in savings and you have three alternatives:

$$a savings account earning $1\%$ interest per year

$$a U$.$S$.$ Treasury bond earning $3\%$ interest per year

$$a stock market mutual fund earning $8\%$ interest per year

Approximately how long would it take to double your savings in each of the three accounts?

$6.$ Assume you plan to retire in $40$ years and are evaluating the three different accounts listed in question $5.$ How much would your $$1,000$ be worth in $40$ years under each of the three alternatives?