Ask Marilyn ® by Marilyn vos Savant is a column in Parade Magazine, published by PARADE, 711 Third Avenue, New York, NY 10017, USA. According to Parade, Marilyn vos Savant is listed in the "Guinness Book of World Records Hall of Fame" for "Highest IQ."
In her Parade Magazine column of (Date Unknown), Marilyn thought it obvious that if a hen and a half could lay an egg and a half in a day and a half, it would only take one hen to lay six eggs in six days. In a followup column of July 1, 1990, Marilyn admitted that the correct answer is one and one half hens.
Thanks to GianPiero Puccioni <email@example.com> wrote to report that this error is discussed in Return of the Straight Dope, by Cecil Adams, Ballantine Books, 1994, ISBN 0-345-38111-4, pages 187-189.
If you have the original column (date unknown), or the followup column (July 1, 1990), please send it or them to me.
Note that the whole-hen and the half-hen are producing eggs in parallel Thus, the whole-hen produces a whole egg while, at the same time, the half-hen produces a half an egg. But both require a day and a half to do it! Thus one hen will produce one egg in one and a half days. Thus, in 6 days, one hen will produce 4 eggs - and hence you need one and a half hens.