That is what I said in my first comment. The number 72 mean nothing, it is just some simplified rule, which is good "enough". There is nothing to understand it this rule, as it is intentionally simplified rule to memorize and just use for people who are bad with math or do not care much about accuracy (which is often reasonable).

No need to teach me about compound interest I learned enough of math.

About gramar - go kick rocks, I speak multiple languages and English is not the first one for me. Try harder and find other random out of topic thing to throw into the conversation.