Parker's latest math.
A while ago, I told Parker about how the digits of the multiples of 9 all added up to nine (ie that 2 x 9 is 18 and 1+8=9 and that 3 x 9 = 27 and that 2 + 7 = 9. He, obviously, thought that was pretty cool.
So today, several weeks later and completely of the blue, he says to me
"Mom, I think the the 9's at 11 and up add up to 18."
"Huh?" I say since only minutes earlier we were discussing waffles.
He repeats himself and then says, "9 x 11 is 99 and 9+ 9 =18."
I said, "Oh, right, but what about 12? Does it work for 12? I don't think so."
So, he thinks out loud, "Well, 9 x 12 is 108 and 1 + 0 + 8 doesn't equal 18. So oh well." Then we walks away and does something else.
But sure enough...if you do the math out and count the first two digits as one digit, he is right.
9 x 12 =108 and ten plus eight is 18
9 x 13 = 117 and eleven plus 7 is 18
9 x 14 = 126 and twelve plus 6 is 18
and so and so on.
I knew the day would come when Parker was smarter than me in math, but I didn't think that day would be today.
Plus...that little dude is sitting around thinking of math more often that I think he is.
3 comments:
Now you got Francis churning the brain cranks. Francis wanted to tell Parker that multiples of 9 add up to 9 if you add up the digits singly. ex. 9x7 = 63; 6+3 = 9. 9x14 =126; 1+2+6=9. Except 11x9 though.
Well, it's not true that the sum of the digits of multiples of 9 upwards of 99 add to 18, even if you read the first two digits as one number.
Counter-example: 21 x 9 = 189, and 18+9 = 27.
What is always true is that if you add all the digits of a multiple of 9, you will get a multiple of 9.
Some examples:
9 x 3 = 27 and 2 + 7 = 9
9 x 11 = 99 and 9 + 9 = 18
9 x 21 = 189 and 1 + 8 + 9 = 18
9 x 7777 = 69993 and 6+9+9+9+3 = 36
It's always true.
Now sure who MD is (maybe Francis), but yeah, I knew it was always a multiple of 9. (And I think he does too.) But he was very specific about 11-20 when he brought up the 18 thing today. I don't think he was saying it worked higher than that. I should have been more clear in the post.
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