It's really interesting watching Art Benjamin do this in his TED talk[0] ... if ...

It's really interesting watching Art Benjamin do this in his TED talk[0] ... if you watch closely you can see him keeping track of the intermediate calculations in his hand movements.

Very clever, and obvious once you know.

[0] https://www.ted.com/talks/arthur_benjamin_a_performance_of_m... ... starts at 7:47