It originally referred to a particular calculator glitch, when using octal mode, which is base 8. Only digits 0 through 7 are supposed to work in octal.
But there was at least one model calculator out there that would still allow you to type in the digits 8 and 9 in octal mode, though those digits should have been ignored and disabled in that mode.
9 in base 10 converts to 11 in base 8, which the calculator automatically does in the background before performing the calculation, so the calculator actually ends up calculating 11 + 10 = 21, which in base 8 is actually correct.
Now how that got misconstrued into memes such as this one, I’ll never know. 🤷♂️
papalonian@lemmy.world 2 months ago
There was a viral video from like 2015 or so (I think it was a Vine) where a guy asks a kid, “what’s 10 plus 9?” The kid looks up and immediately says “21!” to which the original person replies, “ya stupid”.
I don’t think it actually has anything to do with the image, just a post ironic shit post.