Rule of 72

If you are earning a certain rate of interest on your money, divide 72 by that interest rate. The number you get from that division is the approximate number of years it will take for your money to double in value.

I have $5000 and I am getting 2% interest on my money. 72 /2 =36. It will take about 36 years for my money to double to $10,000.

https://accounting.uworld.com/cpa-review/lc/accounting-dictionary/term/rule-of-72/