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Yes, I know this thread is almost a year old, but I thought I might be able to help anyone still struggling with it (as I was until it dawned on me).

I hope this isn't considered inappropriate. All I'm going to say is this: it would be a rather mundane effect if all you were to say was something like, "I have as many as you, plus enough extra to make yours equal to mine," don't you think?

I still love this effect, my treasure chest with gold coins in still works wonders but I should really use 5p so the answer is not 12 coins but 60pence which makes the whole thing much harder to work out.
But you knew that anyways…

The trick is in World's Best Card Tricks by Bob Longe under the title Easy Estimation

Instead of worrying about the math behind trick I like to think of it as Bob wrote in his book - the trick is solely based on the use of words