Incredibly, the great number theorist Ernst Kummer was so inept at simple arithmetic that he often asked students to help him in class. On one occasion, Kummer sought the result of a simple multiplication. "Seven times nine," he began. "Seven times nine is er -- ah -- ah -- seven times nine is..." "Sixty-one," a mischievous student suggested and Kummer wrote the "answer" on the blackboard. "Sir," another one interjected, "it should be sixty-nine." "Come, come, gentlemen, it can't be both," Kummer exclaimed. "It must be one or the other!"
[According to Paul Erdos, Kummer calculated the result using the following logic: "The product cannot be 61, because 61 is a prime number; it cannot be 65, because 65 is a multiple of 5; 67 is a prime; and 69 is too big. Only 63 is left."]