I worked at the IRS way, way, way back. Every night I processed payments I did over 1 million dollars all by myself. It was data entry so fairly-boring; I’d keep a rough running total just for laughs. It was so cold I’d go to work with a sweater on in the middle of the summer.
You’d see checks or money orders for 1 cent or 2 cents. And then you’d see.....
s/b “I’d keep a rough running total in my head just for laughs”