So they just go to the device and press a button and itll cycle through all the choices, then there will be another button to confirm their choice, then the same for the next addend then another button is pressed again to show the final result.
looking from a top view....
LED, LED, LED, LED, LED {Row 1 of LEDs} Store 1st addend
LED, LED, LED, LED, LED {Row 2 of LEDs} Store 2nd addend
LED, LED, LED, LED, LED {Row 3 of LEDs} Store answer (binary)
(button 1 choice button) (button 2 confirm addend) (button 3 shows answer)