“Think of a Number” Trick – how do you do it?

You will have heard of that party trick where someone says, “Think of a number” then they get you to add on something, double it, take away something, add 5, halve it, etc, etc, and then they tell you the number you now have?

WOW! How do they do that? Is it magic? Kind of….. It’s mathematical magic! Let me try to explain.