Mental arithmetic is one of the simplest, most accessible ways to keep working with numbers as a regular habit — no equipment, no login required to try it, and a difficulty level gentle enough to start comfortable and build from there.
Rookie uses numbers up to 9 with gentle pacing — a comfortable warm-up rather than a test. There's no penalty for staying there as long as it feels right.
Difficulty only ever moves up when you choose it — nothing pushes you to a harder level before you're ready.
One or two minutes a day is a genuinely sustainable routine, and the weekly goal is built to forgive a missed day rather than reset a streak to zero.
Playing on the web means there's nothing to install to try it first — the iPhone app is there afterward if it becomes a daily habit.