I recall an anecdote of Isaac Asimov producing a total faster than a store cashier.
My recollection is that when the cashier commented on how this demonstrated Asimov's intelligence he shared that patterns in costs allowed clever abstractions to an easy sum. Asimov shared his technique but the cashier reacted in disgust, saying that it was "just a trick".
I would like to use this story, but I can't find a primary source despite quite a bit of searching. I found a vague reference to a similar story in a book called Speed Math for Kids at the end of page 197, but this doesn't confirm the story to be real and not "just an urban myth".
Does anyone know of a primary source?