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Bob, Carol, Ted and Alice each received three keys in order to work in pairs. On their own they are unable to open all four locks keeping the papers in the cupboard. In order for any two of them to work together, each one had to have a different key missing compared with the others, so one lacked key 1, one lacked key 2, one lacked key 3 and one lacked key 4. It only took any two of them to have all four keys in order to unlock the cupboard. | |
| An example of this would be: | ||
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Bob | key 1, key 2, key 3 |
| Carol | key 1, key 2, key 4 | |
| Ted | key 1, key 3, key 4 | |
| Alice | key 2, key 3, key 4 | |