For the fourth row, set 6 under the 3, 2 under the first 7, and another 2 under the other 7; a 0 under the 9, and 2 under the 7, which complete this row.
You are to do the same for the fifth row, putting 1 under the 8, 9 under the 0, 8 under the 1, 6 under the 3, and 9 under the 0.
Then desire some of the company to cast up these ten sums, and it will be found that the product of the whole addition will form the sum of 499995.
In order to come to this combination, you need only fix the number of figures that will compose each row, and determine the number of rows; then to reckon each row for 9, as has been shewn above.
You may likewise present this addition, by saying, that it is the total amount of ten rows, composed of five figures each; out of which five rows will be set by the person who chuses to do it; then multiply secretly as many times 9 as you are to set rows of five figures; therefore multiply 5 times 9 by 5, which will give you the sum of 499995.
The person having set his numbers, you are to add your five rows, taking care that every number you set will make 9 with that to which it corresponds; which being done, you are to ask any one to cast the whole sum up, and the product will be the same as the sum you set down before-hand.
If it were requisite to employ other numbers instead of that of 9, you should, in order to succeed, warn the persons who chuse to set the figures, to be attentive that their numbers do not exceed that agreed upon.