If you want to quickly ballpark the # of years it will approximately take for an investment to double, then just divide 72 by the annual growth rate. For example, if your investment grows 7% a year, it will take slightly more than 10 years for it to double (72/7). But if it grows 36% a year, it will only take about two years to double (72/36).
You can read more about this “Rule of 72” on Wikipedia. Now, you now you can come across as a math genius without much effort 😉