In the supposition that the sum set down is the following:
789,788
Add in your mind thus: 7 and 8 are 15, and 9, 24; and 7, 31; and 8,
39; and 8 more, 47: in 47 there is 5 times 9, as 9 times 5 make 45;
there remains 2, therefore in order to complete 9, 7 are to be added;
consequently the sum to be multiplied will be 7,897,887.
Then give this sum, which has been increased by a 7, to the person who
has presented it to you: and tell him to chuse whatever multiplier he
pleases; then retire while he does the multiplication, recommending
him to strike out the figure he pleases, as usual, and to set down on
a piece of paper the remaining sum, the figure being defalcated, and
the remaining figures ranged as he pleases; and in order to guess the
number that was struck out, you are to proceed as it has been explained
for the first manner of operating, and with the same tricks.