A person with an abacus can calculate a pearl in his head without using an abacus. This is called abacus-type mental arithmetic (mental arithmetic that visualizes the operation of an abacus), and calculations with more than 10 digits can be performed in a matter of minutes. So why can a person with an abacus be good at mental arithmetic?
This is because the abacus operation in the abacus is the same as the operation of abacus mental arithmetic. The abacus has a minimum combination of 5 beads and 4 beads, so you can easily express numbers from 0 to 9. That makes it easy to visualize the number of abacuses in your head.