The original source is Reusser 1986 p25 (26), who reports that 3/4 of first and second graders give a numerical answer. I learned that from Kaplinsky's 2013 replication, not with eight-year-olds, but with eighth graders (video). He credits Merseth 1993 with popularizing it. Kaplinsky via Gwern.
A late descendent of the joke appears in Science Made Stupid (1986), which I highly recommend.
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