I Did Not Make It Up

In this video, physics professor Albert A. Bartlett explains how the
“doubling time” of anything can easily be calculated by dividing
the number 70 by that thing’s annual growth rate percentage.

So if you are growing at 7% per year, the time to double that population is 10 years = ( 70 / 7 ); if you are growing at 2% per year, the time to double would be 35 years = (70 / 2). If you are growing at 5% per year, the time to double would be 14 years = (70 / 14)