Rule of 72 - does it work for buy&hold properties?

[Samuel Chua]

Hi guys, Ive just heard of the rule of 72 when it comes to borrowing money from lenders. But when i applied this calculation to my property, I realized i will be losing out more money. For example, If I lend 75k and return double the amount in 7.2 years (lets say someone deals with me) and I purchase a property of 70k which produces a cash flow of $513.24 (not including mortgage since I pay everything up front with his cash). This would mean that in 7.2 years, I would have amassed a total of $44,344 in cash flow from my rental property. Now comes the hard part. If let's say I sell my property for $84,700, I will have a total of $129,044 in my account while I have to pay my lender a total of $150k which makes me at a total loss. If anyone could give me advice on this, it would be amazing, thanks!