With diminishing school results in many parts of the modern world, a classic and truly evaluated method to train logic, arithmetic’s and mental capacity is needed. The answer is the Abacus, the traditional Asian method to help visualize and understand numbers and logic.
The strength of using an Abacus frame is its structured way of doing addition and subtraction of large numbers. And when you have mastered it you don’t need the frame itself anymore. You visualize the abacus in your head and do the same structured operations that enables high speed calculations on the frame.
The classic wooden Abacus frame is available in many parts of the world (also 3D-printable versions), but it can now be downloaded as an mental training app on your Android tablet.
Further instructions how it work can be seen on How an abacus work.