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Tax Clarification on Russian Multiplication

In a previous post on the letter of the NIF slipped a small error. What you need to search the table of the letters is the remainder after dividing your identity card number by 23.
The main sticking with that found many people is that calculators are not normally an operation that returns the remainder of a division, decimals calculator out directly.
Windows Calculator, as science (Click on View / Science), can get us out of trouble with the MOD key:
Write the DNI

MOD press the key type 23
4. We click on the = key (or press the Enter key on your keyboard).

Failure to hand calculator windows desktop but a stream is a bit more complicated but can also be done:

1. divide the number of ID card of 23.
2. The result we subtract the part that is left of the comma, ie: the number without decimals. It should be something like 0 ,....
3. Multiply that result by 23. That number is the remainder.

Anyway I do not understand the usefulness of a letter that anyone with a pocket calculator is able to calculate in seconds. What makes a safety extra point is calculated directly from the number?