I know how this works, and you lot will too, but it's a bit of fun that kids will like.
Think of a number
Double it
Add 10
Halve it
Take away the number you first thought of
The answer is..........................5
The answer will always be half of whatever number you say to add
So long as my 6 year old daughter doesn't try to use her standard "300-million-gazillion-billion" I may just have to try this! LOL!