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You have a fox, a goat, a farmer and some carrots. You must cross a river with only two of them at a time. If you leave the fox with the goat he will eat it; if you leave the goat with the carrots he will eat it. How can you get all four across safely?
Take the goat and farmer over first. Go back with the farmer and bring the carrots, but instead of leaving the goat with the carrots, come back with the goat and the farmer. Leave the goat on the first side and take the fox with the farmer. Leave the fox on the other side with the carrot. Finally, go back with the farmer over and get the goat and bring it over.