Okay, here is the answer to how this trick works.

This trick is based on the number 9. For example : if you give me the numbers 123, I would add them up and get 6. I subtract 6 from 9 and get 3. The number you circled would be 3.

If the numbers totaled over 9, say 12, them you would subtract the total from 18 and get 6. And so on with multiples of 9 such as 27, 36, etc.

The only time there is not a certain answer is when the given numbers total 9, 18, 27, etc. Then the answer is either 0 or 9.

There are other tricks using nine as a basis. It's an interesting numeral! Enjoy the magic!