If we read the exact words of the writer in the cyclopædia, we find that
we are not told that the pens were all necessarily empty! In fact, if
the reader will refer back to the illustration, he will see that one
sheep is already in one of the pens. It was just at this point that the
wily farmer said to me, "_Now_ I'm going to start placing the fifteen
sheep." He thereupon proceeded to drive three from his flock into the
already occupied pen, and then placed four sheep in each of the other
three pens. "There," says he, "you have seen me place fifteen sheep in
four pens so that there shall be the same number of sheep in every pen."
I was, of course, forced to admit that he was perfectly correct,
according to the exact wording of the question.