The magical properties of the nine

The human hand is one of the first calculating machines!

Place both hands side by side on the table and extend your fingers. Each finger from left to right will represent the corresponding ordinal number: the first from the left - 1, the second - 2, and so on until the tenth, which will represent the number 10. For example, we need to multiply 7 by 9. Now lift the seventh finger. The number of fingers lying to the left of the raised finger will be the tens of the work, and the number of fingers to the right will be the number of ones. There are 6 fingers to the left of the raised finger, and 3. To the right, the result of multiplying 7 by 9 is 63.

This surprising, at first glance, mechanical multiplication will immediately become clear if we remember that the sum of the digits in each product of the numbers in the multiplication table by nine is equal to nine, and the number of tens in the product is always 1 less than the number that we multiply by 9. Raising the corresponding finger this is what we note and, consequently, we multiply.