The Twenty-One Words
A. Ross Eckler · Digital Commons @ Butler University (Butler University) · 1975
Problem: Using any 21 alphabetic letters of your choice, each one exactly five times, construct a group of 21 five-letter words with the properties that (1) any two of the 21 letters being used will be found in exactly one word, and (2) and two of the 21 words will have exactly one letter in common.