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?
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I would encourage him to keep thinking like this. Not necessarily the way he does math, but the back thought it took to come up with it. What it shows is that he's looking at the systems and synthesizing his own. It's too early to know if he will be a genius, but he shows all the signs of being exceptionally smart.