![Graphic of a calculator, with '7469' on the screen, and number keys arranged in rows: 789, 456, 123](https://aperiodical.com/wp-content/uploads/2024/12/image-3-652x1024.png)
It’s now been a year since I took over the puzzle column at New Scientist and turned it into the BrainTwisters column. By way of celebration, I thought I’d write up an interesting bit of maths behind one of the puzzles, which I made a note of at the time and have been meaning to…