Tell some one to think of any number he likes, but not to tell you
what it is. Tell him then to double it. When he has done that, let him
add an even number to it, which you must give him. After doing this,
he must halve the whole, then from what is left, take away the
number he first thought of. When this is completed, if he has counted
correctly, you will be able to give him the exact remainder, which
will simply be the half of the even number you told him to add to his
own.
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