Add or subtract the same number

Type 34, press , type 57, and press : 91.. The $\displaystyle +57$ operation is retained. Type 45 and press again; it adds 57 once more, giving 102.. Subtraction works the same way. Type 79, press , type 59, and press : 20.. Type 56 and press ; the $\displaystyle -59$ operation is retained, so the answer is −3..

▶ Watch this step in the video (13:18)