I'm going to try to explain this as best as I can. I am still confused about it as he can't explain well. He made this list in his head; 1- 9 2- 8 3- 7 4- 6 5- 5 6- 4 7- 3 8- 2 9- 1 So lets say he's doing 7+3. He'll take the lowest number (3) and turn it into a teen (13). So it would look like 13+7. But then he'll use his list and take the opposite of the other number and replace it. So 7 would be 3 and vice versa. 2 would be 8 and vice versa. So then the problem would look like 13+3. But then he'll flip the sign into a subtraction symbol and do the problem and get the answer. But this formula only seems to work with the numbers between 1-9. So someone tell me if this has been an established formula or if my six year old brother is a genius?