Finally , the operator had to move the slat again to read the correct answer .
The slat was then moved to expose the black digits ( which now displayed the nines ' complement of the minuhend ) and the subtrahend was added by dialing it in .
The operator had to move the slat again to read the correct answer .
The slat was then moved to expose the black digits ( which now displayed the nines ' complement of the minuhend ) and the subtrahend was added by dialing it in .