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.
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.
