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'm not sure I'd encourage this. It's a clever trick (basically adding 10 and then subtracting), but it has its flaws. The main problem being that it only works with numbers 1-10. This is obviously enough for a 6 year old, but if he never learns how to "properly" add and subtract, he may run into issues later. Something similar happened to my cousin when he was younger. He had his own way of subtracting things that worked fine at first, but when he was confronted with larger numbers and more complex problems he had issues coping because his trick no longer worked.