> (for those who don't know too, percentages are reversible so at 10% you double your money every 7 years...)

That is not true at all. "Percentages are reversible" means that 30% of 50 is the same as 50% of 30, so 15. It does not mean that anywhere you see a percent related to another number, you can just switch them around.

Easy counterexamples:

At 100% annual interest, you double your money every 1 year. At 1% annual interest, you double your money every 70 years.

At 41% annual interest, you double your money every 2 years. At 2% annual interest, you double your money every 35 years.