Proof of Multiplication in Ten Seconds.
The unitate of a number is the sum of its digits reduced to a unit.
| 252 | = 9 | } | = 54 = 9 |
| 321 | = 6 | ||
| 252 | |||
| 504 | |||
| 756 | |||
| 80892 | = 27 = 9 | ||
The unitate of the multiplier is 9 and the unitate of the multiplicand is 6; 6 times 9 equals 54, and the unitate of 54 is 9. Now the unitate of the product is found to be 9 also, which is a proof of the correctness of the work.