If you haven't updated your app yet and are still using a version from before September 9, 2018, DO NOT UPDATE. The game no longer works with that update. It will not even register that you are touching the screen. In spite of numerous reports and complaints, the developer has yet to do anything useful about it.
Previous review: Simpler is better! After using a few programs with silly bells and whistles, this is definitely my favorite. It best replicates the comfortable feeling of doing the puzzle for yourself, rather than to necessarily beat the clock or hit a certain score. You have a clock if you want to beat it, but its presence is unobtrusive and pressure-free. You won't get hit by onerous penalties if you touch the wrong number or button (unlike a competing version that I have since uninstalled) and it's pretty hard to hit a wrong button anyway, because everything is arranged so well on the screen. If you DO feel competitive, you can check your position on the leaderboard and keep an eye on the clock, but none of the competitive aspects will ever get in the way of those times when you just want to relax with a favorite number puzzle. Get this this app app and you'll forget all about the others.