Ask some one to start a sum in addition by writing the top line of four
figures. We will suppose he writes 1912. You mentally subtract the 2 and
place it before the 1, making 21,910, which figures write on a piece of
paper, which you fold up and lay on the table. You then ask a second
person to place four figures under the first line. Then add a line
yourself, which must be a deduction of the second line from four 9's.
Ask a third person to add four figures to those already written. Then
add another line yourself, making it a deduction of the third person's
figures from four 9's. Request a fourth person to add up the sum and
tell him you have already done so, and he will find the answer on the
table. The sum will appear something like this:--
1912
7234
2765
4891
5108
--------
21,910
Which answer corresponds with the figures on the paper, which has been
on the table the whole time. If you have in the company two friends upon
whom you can rely as confederates, previously arrange with them to write
the third and fifth lines, explaining to them that they must deduct the
line immediately preceding theirs from 9's and make their lines the
products. This adds greatly to the mystery of the trick.