This trick works by getting you to calculate a number that will always be a multiple of 9. If you pick any 2-digit number ab, and then subtract a and b from that, you are getting (10*a + b) - (a + b), which leaves you with 9*a. The number you end up with will always be 9, 18, 27, 36, etc. The trick to the game is that the "magic code" uses the same symbol for every number that is a multiple of 9. Plus, every time you try again, the site picks a new symbol for all of the multiples of nine. This creates a false impression that the page is somehow able to "read your mind," as the right answer changes every time.
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