How to put 9 pigs into 4 pens so that there are an odd number of pigs in each pen?
Solution 1:
Since this has been exposed, some claim, to be a riddle and not a bona fide math question, why not completely drive the stake through its heart:
Pen 1: 7 pigs
Pen 2: 1 pig
Pen 3: 3 pigs
Pen 4: -2 pigs
Now the astute reader will note that -2 pigs is a pretty darn odd number of pigs to be in a pen!
Solution 2:
It appears to be a trick question.
Make 3 pens, put 3 pigs in each pen. Then put a 4th pen around all 3 of the other ones, and you have 9 pigs in that pen.
Update
@MJD found a source for this problem with solution, see Boys' Life magazine from 1916.