Two wives cross, and one rows back for the third. She drops her off and rows back to her husband. Two husbands cross to their wives and one couple comes back, then both husbands on the near bank cross. The wives ferry themselves over in the last four crossings.
Three jealous husbands and their wives need to cross a river in a boat that holds two people. No wife may ever be with another man unless her own husband is there too, on either bank or in the boat, and anyone in the boat counts as being on the bank it is tied up at. What is the fewest crossings that gets all six over?
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Start by sending two of the wives.
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Eleven crossings
Source: Attributed to Alcuin of York, Propositiones ad Acuendos Juvenes, Proposition 17, c. 800