Manfred wrote: Thanks Pedrinho! I like math problems with a twist... (...)
Manfred / Mathfred
I have a matchbox trick that is awesome. And it involves math... Maybe you are interested.
I will not teach how to do it, but I will try to write how it is shown:
(Before continuing, I have to say that here in Brazil there are two main models of matchboxes. The most common is one that you can open/close like a drawer, in a way that the box itself is made of two separated pieces, and the wooden matches are enclosed in the "drawer" and are not attached to it as they are in the other model. This second model, less common, is like I see on American and European movies, and I suspect that is what you are used to: it's made by a fold paper, and the matches are attacked on it and needs to be detached to be used.
I said that because the trick needs to be made in the first model (the average number of matches that comes in the box is around 40; and the matchbox needs to be hand-shaked, so the matches needs to be free to make noise inside the box while shaking). So, if you don't have it in the USA, never mind...
But if you have one of these, here goes the trick:
I ask anyone in the audience if they have a matchbox (If they don't have one, I give one to them and let them take any amount of matches from it to make sure that I don't know how many matches there are on it before beginning the show).
During the entire "show" I don't see the matchbox, that needs to be hold/manipulated by the audience (except on the final point, when I shake it).
First, I tell them to count how many matches there are inside the box, but don't tell me the result.
Second, I tell the audience to make a simple math operation: they need to sum the two digits that forms the number of matches that are inside the box. If, for example, it was 138 matches inside the box, they need to sum 1+3+8 (=12). They can not say to me this result, all this operations needs to be made in silence.
Third, I ask them to take this number (the sum, in this case, 12) of matches out of the box, leave the rest in there and close the box.
Fourth, I take the box with one hand, shake it near my ears, and tell them how many matches are left inside the box.
The beauty of this trick is that I can only know how many matches are left in the box by shaking it! Believe me: if I don't shake the box, if I can not hear the noise of the matches, I can not know the result!!!
Your task is to find out how this trick works. It needs to be said that it involves some skill (because I
really need to shake the box), but it has some interesting math facts involved...