We evaluate an educational system, "SoroTouch" developed for the acquisition of abacus based mental calculation skills efficiently. SoroTouch is an iPad app with a unique interface called "Invisible mode", where the beads are not displayed, and the student is required to visualize and operate the beads mentally to solve the problem. Furthermore, SoroTouch helps to train fast accurate management of the beads, while providing students with features to facilitate continuous learning. The analysis of user learning data revealed that in 2017 and 2018, 54% and 62% of withdrawing students successfully acquired mental calculation abilities by the time of their withdrawal, respectively. Time spent on SoroTouch for skill acquisition was verified to fall between 7 and 26 months. This time period is defined from the student's start of learning, until the student's demonstration of mental calculation abilities.