Ernst Kummer was a German algebraist, who did some of the best work on Fermat’s Last Theorem before the modern era. However, he was poor at arithmetic, so he always asked his students to do the calculations for him. On one occasion, he needed to work out 
“Sixty one” suggested one student. Kummer wrote this on the blackboard.
“No, Professor, it should be sixty-three” suggested another student.
“Come, come, gentleman,” said Kummer, “it can’t be both. It must be one or the other”.
More later,
Nalin Pithwa
Mathematics Hothouse 