Take Any Number ...

This is actually the recipe for most of the movies these days.

Did you know... If you take any number, double it, add "any second number", divide by 2, and subtract the first number, the answer is always "half the second number. " Like any magic trick it disguises the actual math (so to speak) with distraction math (the audience participation, choose a number)

Actual math - 10/2 = 5
Distraction math - (#x2)/2-#=0

: )

SubStract?

Just as math gives you the tools to make things work, ultimately some mathematician will give you a tool to break things by removing assumptions. Try it out with modular arithmetic with some careful choice... like the value 2 mod 4:
2*(2 mod 4) = 0 mod 4
(0 mod 4) + 10 = 2 mod 4
(2 mod 4) / 2 = 1 mod 4 (definition of division in modular arithmetic is tricky)
(1 mod 4) - 2 = 3 mod 4

Although modular division is kind of tricky, and there are two possible values x, such that 2x = 2 mod 4: the 1 mod 4 used above, and 3 mod 4, for which 3 mod 4 - 2 = 1 mod 4 = 5. So it can still work, but for only one of the possible outcomes of the division.

Not if you can't remember your original number!