A spectator is ask to think of any number. Then, the the spectator is asked to add 5, add 10, subtract 7 and add 15. After that, he/she is asked to subtract from the number that he/she choose. The answer will be 23. This is because
Let's say the spectator chooses 8. 8+5=13 13+10=23 23-7=16 16+15=31
So, it is a simple trick with mind-blowing response.