>>> Most people would consider that you need a calculator to do any
>>> computations in base 10, beyond really trivial ones.
>>
>> Trivial, like multiplying 2 random phone numbers in your head? :P
>> I've done that several times, just for the /fun/...
>> Take some random 10 digits number, then another random 7 digit number.
>> OK. Now, without paper, what is the product? Hint, it's 17 digit long,
>> possibly 18. Your pocket calculator can't give you the exact result.
>
> Now, personally, I can't even *remember* a 17 digit number without
> pencil and paper, never mind *compute* it. :-P
>

Remember the 10 digit number and the 7 digit number.
Multiply the first one by the first digit of the 7 digit one.
Remember that.
Repeat for all other 6 digits.
Shift and add the 7 intermediate results.
DONE!

It's a king of mental jogging :)

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