A puzzle from Alcuin of York’s Propositiones ad Acuendos Juvenes (Problems to Sharpen the Young), 9th century:
“A man and his wife, each the weight of a loaded cart, who had two children each the weight of a small cart, needed to cross a river. However, the boat they came across could only carry the weight of one cart. Let him devise [a way] of crossing in order that the boat should not sink.”
(The boat can bear the weight of two children or one adult.)
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Also in the same manner, first, the two children get in [the boat] and cross; one of them then brings the boat back. Then the mother gets in the boat and crosses; her son brings the boat back. With the boat back, the brother of this one gets in the boat and both cross; one of them then brings the boat back to the father. When the boat has returned and with the son on the bank, the father may cross. Then the brother who had gone across before get in the boat and brings it back to his brother. Now with the boat returned, both brothers get in and cross.
“By such a clever plan of crossing, the navigation can perhaps take place without the boat sinking.”
(Peter Burkholder, “Alcuin of York’s propositiones ad acuendos juvenes: introduction, commentary & translation,” History of Science & Technology Bulletin 1:2 [1993].)
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