A THIRD WAY.
Ask the person to add 1 to the triple of the number thought of, and to multiply the sum by three; then bid him add to this product the number thought of, and the result will be a sum from which if 3 be subtracted, the remainder will be ten times of the number required; and if the cipher on the right be cut off from the remainder, the other figure will indicate the number sought.
Example—Let the number thought of be 6, the triple of which is 18; and if 1 be added, it makes 19; the triple of this last number is 57, and if 6 be added it makes 63, from which if 3 be subtracted, the remainder will be 60; now, if the cipher on the right be cut off, the remaining figure, 6, will be the number required.