Alcuin's hungry camel: can 90 measures of grain survive a 30-league walk?
Alcuin of York's 9th-century puzzle collection has a man ordering 90 measures of grain moved 30 leagues to another house. He has only one camel, strong enough to carry 30 measures per trip — but the camel eats one measure of grain for every league it carries a load. The grain must go in three separate loads before the final push to the far house. How much grain is left when it finally arrives?
Reveal the answer
Only 20 of the 90 measures survive. The camel ferries the grain in three loads of 30 measures each, 20 leagues forward to a way-station — each load shrinking to 10 measures along the way, since it eats one measure per league carried. That leaves exactly 30 measures pooled at the way-station: one final full load, which the camel carries the remaining 10 leagues, eating 10 more, arriving with 20. Written around 800 CE, this is now recognized as the earliest known ancestor of the 'jeep problem' in operations research — the same fuel-relay logic used for desert crossings, first formalized mathematically by N. J. Fine in 1947.