An old-fashioned skill... For about half my mom's life, the cash registers used by check-out clerks at grocery stores and most other retail stores were mechanical devices. My younger readers may have trouble believing this, but bar code scanners had not yet been invented. The poor clerks had to manually enter every single price into their mechanical cash register, which added them together using a complicated set of gears, levers, and so on. The total showed up in the form of a miniature billboard for each digit. Such a thing is almost unimaginable these days!
At least up until the early '70s, when I left home to join the Navy, my mom was able to do something completely unique in my own experience. As we loaded up her shopping cart in the grocery store, she would mentally add together the cost of all the items in the cart. So far as she was concerned, that cash register (and the clerk) were there to cheat her out of her money (something she never had much of), so she would double-check them. It was fairly common for her to catch an error, too – I'd guess it was maybe 20% or 25% of our shopping trips. Somehow more than half the time the error was in the store's favor, too :) Each time her mentally-computed total didn't match the total on that cash register, she'd make the clerk go over the whole thing, using the tape generated by the cash register (which just had prices, not the name of the item). Sometimes the error was an incorrectly entered price, and sometimes it was a mismatch between the price marked on the shelves and the price the clerk looked up. In every case, mom would insist on getting the error fixed (even when it was in her favor) before she'd hand over any money.
When we were grocery shopping, she was buying for six people (she and my dad, and four kids). There would be a lot of different items in that shopping cart – and yet, most of the time, her mentally-computed total matched the clerk's total to the penny. Can you do this? I certainly can't!
So how did she do this?
I suspect her ability to do this sort of arithmetic was somehow related to her ability to remember the cards played during a game. Both of them seem to me to be astounding feats of memory – and in both cases, she dismissed them as simple things that any idiot should be able to do effortlessly.
In the '80s and later, when I went shopping with her, I noticed that she no longer double-checked the clerk. When I queried her about this, she told me that she could still do that mental arithmetic, but it was no longer worth the effort, because she hardly ever caught an error. I suspect part of her dropping it was also the fact that money wasn't quite so tight with she and my dad any more. About 10 years ago, I asked her again, and she told me she'd lost the skill – but wasn't going to put the effort back into re-learning it.
It seems somehow a shame that people generally no longer have this mental arithmetic skill. Once this was a common skill, but today someone with this skill looks like a magician performing tricks. On the other hand, I'd trust my calculator's answer more than any answer a human produced :)