For the past couple of weeks, I’ve been obsessively playing the game *Twenty* on my phone. The fact that my wife has consistently been ahead of my high scores has nothing to do with it.

*Twenty* is another in the current spate of “numbers-in-a-grid” games that also includes *Threes*, *1024*, *2048* (and its $2^{48}$ clones), *Just Get 10*, and *Quento*.

The basic idea is that you have a grid of numbered tiles, and you combine them to build up your score. While there are lots of unimaginative derivatives of the bigger games, there’s a surprisingly large range of different games following this template.

With so many different games being created, I thought that a chap like me should be able to come up with a numbers-in-a-grid game of my own. Yet, for a long time, I just couldn’t come up with anything that was any good.

Yesterday I had a really nice shower, and the accompanying feeling that I’d come up with a really good idea – make a game to do with arithmetic progressions.