CARDS THAT CAN GUESS NUMBERS

 From Grolier's 1941 edition of the The Book of Knowledge:

        If we take six visiting cards, and copy on them the six sets of figures that are printed below - one set on each card - we shall be able, with the six cards that we have made, to tell any number that a friend may have thought of.

       We ask the friend to think of a number, and then we show our six cards, desiring that he will point out on which of these cards the number thought of appears.  In a moment we tell him the number, much to his astonishment. 

       The explanation is very simple.  We merely add up the figures that appear on the top right-hand corners of the cards upon which his number appears, and the total that results is the number thought of.  Thus, suppose that our friend thought of the number 47.  It is on cards A, B, C, D and F.  The figures in the top right-hand corners of these are 32, 1, 4, 8 and 2, which, added together, make 47 - the number thought of.  We can guess people's ages in this way, and can get a great deal of fun from so simple a set of figure cards.

       Some people know of figure cards by which a number can be guessed, but these are usually arranged so that the top left-hand figures have to be added to give the solution, whereas here the right-hand numbers must be added.  If any of our friends have seen figure-guessing cards before, they will at once say that they can do the thought-reading as skillfully as ourselves.  We let them try, and by adding up the left-hand figures they find that they are wrong every time.

 

 

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