I got this one from a Usenet archive on math problems which can be accessed
here,
look under logic problems, part 2.
MathProblems.info home
h=human
b=big monkey
s=small monkey
1. Row b and s over ( hhhs / bs )
2. Row b back ( hhhbs / s )
3. Row b and s over ( hhh / bss )
4. Row b back ( hhhb / ss )
5. Row h and h over ( hb / hhss )
6. Row h and s back ( hhbs / hs )
7. Row h and b over ( hs / hhbs )
8. Row h and s back ( hhss / hb )
9. Row h and h over ( ss / hhhb )
10. Row b back ( bss / hhh )
11. Row b and s over ( s / hhhbs )
12. Row b back ( bs / hhhs )
13. Row b and s over ( / hhhbss )
Here is a remark on this problem I received from Rob Pratt,
Department of Mathematics
of The University of North Carolina at Chapel Hill,
"There are exactly three other solutions of the same length, and none
shorter. It probably doesn't make any sense to explicitly list them all,
but you might mention that there are exactly four shortest solutions."