Feynman actually touches on this in the same chapter, a few pages before the anecdote quoted in the article: > I had a lot of fun trying to do arithmetic fast,

Feynman actually touches on this in the same chapter, a few pages before the anecdote quoted in the article: > I had a lot of fun trying to do arithmetic fast, by tricks, with Hans [Bethe] [...] He was nearly always able to get the answer to any problem within a percent. It was easy for him—every number was near something he knew.