It wasn’t a big grocery run - in those days, I was a single bloke, living alone; a loaf of bread, a frozen pizza, a jar of coffee… “Dinner,” as I called it.

Oh and a few Lindt balls. You know them - the little chocolate balls you get at Christmas, with some sort of tasty filling. I liked the white chocolate ones, but I’ve cut down. They’re about the only edible chocolate on sale in America.

“Beep,” said the checkout; “did you find everything you were looking for?” said the lady behind it. I nodded to both.

“That’ll be \$47,” said the checkout lady, and I scratched my head.

“Are you totally sure about that?”

She pointed at the screen. Sure enough, it said \$47. I put away the \$20 bill I had prepared and got my debit card out.

“Sorry to be a pain,” I said - one of the benefits of having a British accent in the US is that you can get away with a spot of the Hugh Grants - “but could I see the receipt?”

Why was I making a fuss? Well, it was because I’d run the sums. I knew perfectly well that even the top-of-the-line coffee I’d picked wouldn’t have taken the total over \$20. Three bucks for the bread, six for the pizza, seven for the coffee, one for the chocolate, give or take. Ever since I was a flat-broke student, I’ve always had the habit of keeping a running total, to the nearest pound or dollar or so, as I go around shops - it helps me to have the right money ready.

“What are these three \$9.99 items?” I asked. My shopping was still clearly visible in the bagging area. There were no \$9.99 items to be seen, let alone three. The checkout lady’s face reddened.

“The Lindt balls are three for a dollar, right?”

I’m not sure how the chocolates had rung up at \$9.99 each. Perhaps it was a system error on the till (more than likely; this particular supermarket was still getting to grips with electricity, so computers were a bit of a leap.) In any case, the lady apologised profusely, corrected the mistake and revised the total to \$18.03.

“There’s \$20,” I said smugly; basic maths skills had saved the day again.