Adding and taking away is Carpenter’s figure addition or subtraction, the course’s combining: the third cell is made from the first two. Union keeps everything, intersection keeps what both share, exactly-one keeps what is not shared, difference removes one from the other. The bank builds each two ways: with the parts kept visibly separate, and with them superimposed into one figure.
What you see: two cells that look like ingredients and a third that looks like their result. What you do: name the operation on a complete row, confirm it on another, apply it.