You will need:
- A brain
- the ability to do simple addition and subtraction
Step 1: Step 1
Step 4: Step 4
Step 5: Step 5
Step 6: Step 6 finally!
Step 7: The Trick
The secret lies in Step 3 where you ask them to add a number.
You can pick any number divisible by 2 to be added. The final answer will always be the number you asked them to add in Step 3 divided by 2. In this case I asked you to add 10 to your total, hence your final answer was 5.
Good luck and have fun with this.