New York Times:

The caller read out the numbers at a speed evoking an auctioneer on fast-forward, each multidigit figure blurring into the next.

Within seconds, Daiki Kamino’s right arm shot up in the air, triumphant. Not only had he heard every number, he had tabulated them and arrived at the correct, 16-digit sum: 8,186,699,633,530,061.

He did it all on an abacus.

I’ve since lost the knowledge but I was taught as a kid how to use an abacus. It was fun and actually fairly easy for ordinary math problems.