This site claims to read your mind!. Yes it can do it even when I’m wearing my tinfoil helmet. Go and try it. The maths to how it works is fairly straightforward as I’ll explain over the fold, but at first glance I must say it is confusing enough to be suprising.
You can write each 2 digit number as 10a+b, so when you deduct the sum of the two digits (a+b), you get:
10a+b – (a+b) = 9a. So you are restricted to a small set of results.
If you look every 9a, ie 9, 18, 27, etc have the same symbol, which is what appears in the crystal ball. Confusing enough for senior management at where I work to send a querry to their pet geeks…
At least they didn’t believe it really was reading their minds.