I recommend that you make a little "cheat sheet" and carry it in your purse/wallet. Write on it how to go about figuring out the rate of interest on a loan and then use it to figure out the interest rate on anything BEFORE you sign on the dotted line.
Make it as simple or complicated as you need, it's your tool for you to use so it makes no difference if it doesn't make sense to anyone else.

I'm glad that you're concerned about making sure you get the best deal and don't get taken advantage of. Good luck.

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Jesus SAVES by shopping smartly and using double coupons!