system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How was The Rule of 70 discovered/derived?

So both in population growth and investment, The Rule of 70 is used to calculate the doubling rate (of a population or an investment). While I found out how to apply the calculations, how was the number 70 found to work?
gpt
The natural log of 2 is 0.693.  It's a lot easier to remember 70.  Lots of folks use 72 (because it has more factors), which is often close enough.

The real formula is T = ln(2)/ln(1+r).

The ln(2) is for "doubling", you could use ln(3) for tripling.

If you work in percent you use 9% = 0.09, so rather than do that and have a rule of 0.70, the .693 gets multiplied by 100, so you look at 70/9 .