First you need to get a partner. Tell them to think of a number between 1-10 but don't tell you. Ask them to multiply that number by two.
Now comes the trick part. Ask them to add an even number to the number they got when they multiplied. It has to be an EVEN number.
Now tell them to divide that number by two. Then ask them to subtract it by the number they originally started with. The answer is always half of the even number you asked them to add.
In an equation , it would be like this. Let's say you start with 5. Then it would be 5•2+6\2-5=3. 3 is half of six. Also you don't have to add 6. U can add any number u want. :)