Thank you, Grandpa

My grandpa collected old coins or maybe he just kept a bunch he had when he was a young man. Grandpa has been gone for over a decade, but I recently came into the possession of those coins. I took some coins that had a face value of $6.10 to a coin shop. I walked out of the shop with $65.00! Yes, that's how much I sold them for.  Grandpa knew what he was doing! That's a better return then a savings account! So maybe one of you math experts out there can tell me the rate of return or the interest rate or the percentage of profit. I don't do math, but wow!