Back to the same number 7

This maths trick to me one of my friend. It is very funny!
It is very simple:

Ask your friends to remember keep in mind any number. It is always better a two digit number if don't have pen of pencil and paper with you. Now ask him to double that. Add 5 to it. Add 12 more to it and subtract 3 and divide by 2. Now subtract the original number  which he was remembering his mind from the above result. Now see the result! it is always 7.

with one example we see how this work!

[first we did] - Let us consider the number: 10

[second we did] - Doubling it, we get: 20

[Third we did] - Now adding 5 and 12 successively, we get: 25 & 37

[Fourth we did] - Subtracting 3 from the above yields: 34

[Fifth we did] - Dividing the above number by 2 would make: 17

[Six we did ] - Subtracting 10 (which was the original number) from the above gives: 7

How funny! enjoy!