*How to become a Lightning Calculator by Anonymous is part of the HackerNoon Books Series. You can jump to any chapter in this book here. Proof of Multiplication in Ten Seconds.*

## Proof of Multiplication in Ten Seconds.

The *unitate* of a number is the sum of its digits reduced to a unit.

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.

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