Hardly on the spot, either, and I'll wager you weren't devoting a lot of clock cycles to other activities (like, say, hosting a game show) while you did it, either. Also, adding and multiplying are easier than division and subtraction.
All I have is my word. I was alphabetizing and collating payments while doing the math.
I remember watching the Grand Slam Grand Final with you, and being able to keep pace with the finalists on the mental math. But that's something I've been practicing for about 25 years, so I should be able to calculate a fifteen percent tip without my fellow diners looking at me as if I've just grown horns and a tail. And yet, that's most often the reaction.
/I am not implying that reciting the alphabet and looking for checks that don't have signatures on them is more difficult than hosting a nationally broadcast game show.
//Yes, there are scoreboards that are updated as the game rolls along.
///You thought Pat Sajak was able to tell players how much money was left to shop with just because of his mad skills? Haw.