The Rule of 72 is used to estimate the time for an investment to double,  You take 72 and divide it by the interest rate as an integer to get an estimate for the doubling.  For example, if you have a growth rate of 3%, 72/3 = 24, so with a growth of 3%, your investment will double in approximately 24 years.

But the rule also works growth rates other than investments.  If the population is growing by 3% per year, the population will double in 24 years.  Another example is medical spending in the U.S., which is growing at approximately 9%, meaning medical spending doubles every 8 years. The rule is just a rule, but what isn't happening is a rational discussion about the natural consequences of the growth rate.  If 9% per year translates into doubling every 8 years, the discussion should be on the core issue, the growth rate, and not on the side issues.