Here's why it's a flaw and why it will make you look stupid every single time:
Step 1) Tell the woman you want to play a game, have a trick, etc. She accepts. You tell her to think of a number.
Step 2) You start making her do lots of mental math. As much as I'd like to think most people can do this math properly, sadly there are a LOT of people out there who will screw something up during this process. This is one flaw - the end-result will not be equal to 6.
Step 3) If, by chance, the math is done properly, you'll tell her that her new number is 6. "Yay! That was so cool!", she'll cheer. "Do it again!" Oh wait, the number is 6 every single time. If she's quick enough to do the basic math involved with this trick, she's quick enough to realize that the answer is 6 under all circumstances.
I'm not bashing you, I'm simply posing a counterargument. Isn't that how new ideas are born anyway? Be more open-minded yourself.
Edit: the guy above me beat me to the punch, and I'm not surprised to see that we both summed it up similarly
