A farmer has to transport a fox, a goose and a sack of grain across a river. The boat is so small that there is room only for him and one of the others at a time, but if he leaves the fox and goose together the fox will kill the goose, and if the goose and the sack of grain are left together the goose will eat the grain. How does he get them all over?
1st journey takes goose only, leaving fox and grain. Returns leaving goose on other side.
2nd journey takes fox over, leave fox on other side, returns with goose.
3rd journey takes grain over leaving on the other side with fox. Returns and picks up goose again taking it over on the 4th journey.
Alternatives (for those smart arses):
A2: Geese can swim so takes fox over first goose tethered and swimming behind each journey. Returns and picks up grain (2 Journeys).
A3: Fox and grain in boat, both farmer and goose swim, farmer pushing boat (1 Journey).