The result may be easily known, since it will be the
half of the number given in the third part of the operation; for
example, whatever be the number thought of, the remainder will be 36
or 25, according as 72 or 50 have been given. If this trick be performed
several times successively, the number given in the third part of the
operation must be always different; for if the result were several
times the same, the deception might be discovered. When the five first
parts of the calculation for obtaining a result are finished, it will
be best not to name it at first, but to continue the operation, to
render it more complex, by saying for example: "Double the remainder,
deduct two, add three, take the fourth part," etc.; and the different
steps of the calculation may be kept in mind, in order to know how
much the first result has been increased or diminished. This irregular
process never fails to confound those who attempt to follow it.
ANOTHER WAY
Tell the person to take 1 from the number thought of, and then double
the remainder; desire him to take 1 from this double, and to add to
it the number thought of, in the last place, ask him the number arising
from this addition, and, if you add 3 to it, the third of the sum will
be the number thought of.
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