When you are unavailable to work in your head, for instance 1+1=2 No need for a calculator as that is a no brainier, now try $12,001,615,817.99 subtract a sum amount of $2,278,726.71 and then multiply by 12,000.
Answer:
all math past 6th grade.
Explanation:
that's when math starts to get really difficult and the process of "showing work" takes too long nowadays, so teachers schools etc even students buy calculators.
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