This app is an ideal tool for learning how to operate an abacus.
For inspiration how to add, subtract, multiply, divide, and square numbers with it, have a look at https://www.youtube.com/watch?v=j32KcnfAH_o.
A base-10 abacus takes some screen space. You might want to hold your device in landscape orientation. All recent devices (size > 720 pixels) are supported.
No ads, no in-app purchases, no bells and whistles. Ideal for kids.