Think of a number

In my school playground there was occasionally a bright spark who would say "Think of a number" "Don't tell me what it is". He or she would then ask you to double it, divide by 2 and then take away the number you first thought of etc etc etc I'm sure you have all been subjected to this kind of stuff.
Think of the year in which you were born. Don't worry about the century. For me 1950. Just call it 50.
Then add your age this year. In my case 61 (Dec 18th Gentle reminder)
So for me 50 and 61 make 111. The funny thing is that yours, whoever you are, however old you are or whenever you were born; yours will be 111 also!!! It's called....................Maths.