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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This is just a really round about way to complete an extremely simple math problem. If it works for him that's fine, but this is not new or mind blowing, sorry.