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Comment by gus_massa | Hacker News Reader

From https://www.jstor.org/stable/27962023

    1 ->  7%
    2 -> 24%
   19 -> 28%
   21 -> 27%
  IDK -> 14%    "I don't know"

At least they know the answers is not 1 (perhaps they thought the answer of a sum in never 1???), but ignoring the 1, it looks worse than random.

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Linking to the cited followup study that ran 36 years later. Similar results are reported. https://files.eric.ed.gov/fulltext/ED565462.pdf

Well if these issues are as universal as they say, continuing across decades and very different cultures and educational systems, then it seems likely that it's just an inherent fact about humans that only about a quarter of them are able to understand the concept of rational numbers.

Well at least that provides more information than the OP's title or link, but since the paper is behind a paywall there's not much more I can say about it.

It is odd that 1 got the smallest number of responses, since it's by far the second best answer.

If you use a bad summation algorithm, 12/13 + 7/9 = (12+7)/(13+9) = 19/22, so 19 will be probably a common answer.

I can't imagine how to get 21. Perhaps 12/13 + 7/9 = (12+9)/(17+7) = 21/20 ??? It's somewhat like a bad version of a division, but I don't expect it to be so popular.

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