A million Japanese children every year learn the abacus, which they call soroban. The high point of the abacus calendar is the All Japan Soroban Championship. And the high point of the championship is the category called Flash Anzan'. It equires contestants to use the mental image of an abacus.
In Flash Anzan, 15 numbers are flashed consecutively on a giant screen. Each number is between 100 and 999. The challenge is to add them up. Simple, right? Except the numbers are flashed so fast you can barely read them.