Proof of Multiplication in Ten Seconds.
Reading edition
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.