I can read your mind

The rules of Algebra even allow us to read the mind of others. Try this one:

1.) Choose any number: (Let's say we try 6) ==== > x
2.) Double this number: (=12) ===============> 2x
3.) Add 4: (=16) =========================> 2x + 4
4.) Divide by 2: (=8) =======================> (2x + 4) x 1/2
5.) Add 13: (=21) =========================> x + 2 + 13

After hearing the result (in our example it is 21) we can immediately figure out that the original number was 6. How do we do that? There is by no way a David Copperfield stunt or any other supra-natural power involved. It is another simple application of algebra.
Just look at the operation in the 5. step. It can be simplified to:
x + 2 + 13 = x + 15

In subtracting 15 from the answer we always get the original number which reduces the ability to read the minds of others but increases the application of algebra.