Tell a person to think of a number, multiply by 3, multiply the product by 2, divide the result by 6, add 20, subtract the number thought of, divide by 4, and then tell him what his result is.

Method. The result will be five. The reason is clear. By multiplying by 3 and 2 and dividing by 6 he has obtained the number thought of. Add 20, he has the number thought of, plus 20; then subtract the number thought of, and he has twenty. Now I know he has twenty; hence, I can tell him what he has if he divides by 4.