10+9=21

If you haven’t heard the latest meme going around, it’s a video of a kid saying 9+10 is 21  I won’t go on about the video, but in my class at the moment the kids think it’s funny to respond with “21” to just about any question.

monadnock
Anyway, on Friday we took our first field trip of the year to climb Mt. Monadnock (above). It was a beautiful day, and everyone had a great time. Now, there’s nothing like getting to experience a group of 8th grade boys chatting on a two hour Friday afternoon bus ride.

See if you can figure out the pattern:
“What’s 9+10?   21!”
“What’s 22+22?   46!”
“What’s 15+20?   37!”

Then we pushed our knowledge a bit:
“What’s 15/5?   5!”
“What’s 5×5?   27!”

I was hoping to get a light to turn on that inverse operations didn’t work in our number system, but had no such luck, and didn’t feel much like pushing them on a bus ride back from a field trip.

Then a student asked me if I would still give him credit if he started writing his answers in class this way. I told him I would, if he was consistent 100% of the time. Then I challenged him to see how well he understood his new number system:

“What’s 3+5+7?”

This one took a minute, then several kids shouted out “17!”

“But 3+5 is 10, and 10+7 is 19. shouldn’t 3+5+7=19?”

This created some excellent discussion.

I’m skeptical that any meaningful math learning happened during our conversation, but I’m happy that we were able to laugh and joke about math outside of school. I hope my students appreciated that as well.

8 thoughts on “10+9=21

  1. Sarah Hagan (@mathequalslove)

    So, my students have decided that I say “21” just like the boy in the video. They keep coming up with random questions to ask me in class that have the answer 21. At first, I had no clue what they were doing, so I would answer their questions. What’s 7 times 3? 21. The whole room starts laughing. Okay. This is not weird at all.

    Then, we took a student council trip this past week. Some students told another student about how I supposedly say “21.” She yells from the back of the bus, “Ms. Hagan! What’s 11 plus 11?” “22.” “Oh! Oops… Uhmmm… I meant to ask: what’s 10 plus 11?” “21.” Again, uncontrollable laughter from the entire bus load of kids.

    Glad to hear my students aren’t the only ones obsessed with this video…

    Reply
    1. Shel Stone

      Using your logic almost all numbers are not their real total then. 40+9=49 would be 20*2 =40/ 3*3=9. So by using your logic 20*3=60 + 2*3=6 so the answer is 66. Humanity is doomed!

      Reply
    2. smart kid

      Actually it would be this:
      2*5+3*3
      Because of order of operations, we would have to do the multiplication first.
      The way you word is as if you could swap around all the numbers, but you can’t, it would have to be all multiplication in order to use the commutative property

      Reply

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