Locks and keys so any three of five can open
Question
Five people share a safe. They want any group of or more of them to be able to open it, but no group of or fewer. The safe gets several locks, all of which must be opened; each lock may have several copies of its key, but a key fits only its own lock. What is the minimum number of locks, and how many keys does each person carry?
Short answer
locks; keys each. Every pair must be shut out by some lock whose keys they both lack, and that lock's keys must be held by all three others; distinct pairs need distinct locks, giving locks with keys each. The keys spread evenly, per person.
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