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 always had trouble adding numbers past a multiple of ten (so like 66+7) so the way I do it is to figure out how much I need to get to the next one (so 4) and then figure out what I have left and then add it on (3, so the answer is 73). If it's larger numbers like 47 then I'll add the tens and do the same as above once I've only got single digits to worry about. Different minds work in different ways.