At School I Was Hopeless At Mathematics And Algebra,

take this easy to follow equation, 

(2 x + 10) - x

2

(x + 5) - x

 as my maths teacher would say, so easy a 5 year old can understand it, which was not very encouraging as I was 11 at the time, so what does the equation mean?

so here it is, think of any number double it, add 10 divide by 2, and subtract the original number, and the answer will always be 5, but why?