Ernst Kummer's 7*9 anecdote

Prussian born Ernst Eduard Kummer (January 29, 1810 – May 14, 1893) was one of the greatest mathematicians ever lived.

Despite being called the father of modern arithmetic (that is, number theory), Kummer was rather poor at simple arithmetic. Once, in a class, he needed to find the product of seven and nine. “Seven times nine,” he began, “Seven times nine is er – ah --- ah --- seven times nine is ….” “Sixty-one,” a student suggested. Kummer wrote 61 on the board. “Sir,” said another student, “it should be sixty-nine.” “Come, come, gentlemen, it can’t be both,” Kummer exclaimed. “It must be one or the other.” Kummer then calculated 7 x 9 as follows: “Hmmm the product cannot be 61, because 61 is prime, it cannot be 65, because 65 is a multiple of 5, 67 is a prime, 69 is too big. Only 63 is left.”