Nobody would ever accuse me of being a math whiz, though I do feel I have skills some of my younger acquaintances lack: I eschew a calculator on occasion to make sure the various cortices of my brain responsible for addition, subtraction, multiplication, and division get a little workout. You just never know when your battery may run down while you’re stranded on a desert island and have an immediate need to balance your checkbook. Oh, and there’s another mathematical task that I’ve mastered. An older friend told me not long ago that physicians who are concerned that a patient may be in the early stages of dementia will test mental agility by asking said patient to subtract backwards from 100—by 8s. I’ve gotten quite facile at that effort—and have moved on to 7s with similar success. Why am I telling you all this? Because I’m inviting you to join me in exploring a story that involves number theory—a deep dive that I have no business whatsoever attempting. But my friend Allan, who excels in math, knows I like quirky stories and thoughtfully sent me this one, which appeared in Popular Mechanics.