This Android one is free and very responsive. https://play.google.com/store/apps/details?id=br.net.btco.soroban&hl=en https://play.google.com/store/apps/detail

This Android one is free and very responsive. https://play.google.com/store/apps/details?id=br.net.btco.soroban&hl=en https://play.google.com/store/apps/details?id=br.net.btco.so... There are Youtube videos explaining the algorithms. A basic exercise is just counting, and then do a countdown. And then try to do it as quickly as possible. Then do it in intervals of 2 rather than 1, then 3, and so on.