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A man has to get a chicken, fox and corn sack across a river. He has a rowboat, and it can only carry him and one other thing. If the fox and the chicken are left together, the fox will eat the chicken. If the chicken and the corn sack are left together, the chicken will eat the corn.
What is the man rows the chicken over the river and leaves him there, comes back for the corn sack, rows over with it, but swaps the chicken for the corn, chicken goes back with the man, then you row the fox over, and leaves the fox, and finally, you come back for the chicken and rows him across the river?