No Magic in Maths

There is no magic about maths. If you think maths is magic, it is only because you do not understand. Take this puzzle, one of a kind often told in primary school.
Think of a number.
Double it.
Add 10..
Halve it.
Take away you original number.
Is your answer 5?
Of course it is!
To see why this works and why it does not matter what number you thought of, left the number you thought of be called  .
When you double it you have  
When you add 10 you have  
Halving gives  
Subtract you original number    gives 5.