Then Mentor takes one more marble, shows it to Cecali and then adds it to the hidden marbles. I had some marbles in my hand, but you do not know how many. Now I add one more to them. Mentor opens the hand that holds marbles. If the student does not answer within a minute, Mentor repeats the same procedure without any explanations.
Mentor demonstrates it with marbles.
Now let us try one more. Mentor gives an example of the same sort. Cecali and Mentor take a couple of turns doing that. Let us put it on paper. Mentor shows with marbles, pointing at the corresponding parts of the formula on paper.