Add or subtract the same number

Type 34, press , type 57, and press : 91.. The addition of $\displaystyle 57$ is kept. Type 45 and press again: it adds $\displaystyle 57$ once more, and you get 102.. Subtraction works the same way. Type 79, press , type 59, and press : 20.. Type 56 and press : the subtraction of $\displaystyle 59$ is kept, so the answer is −3..

▶ Watch this step in the video (14:07)