My niece once asked my dad something, I don’t remember what, and he said something to the effect of “Well, there are six of them, doing it seven times, so just multiply six by seven.” My niece gave him a quizzical look, then reached for a calculator.
Probably the only time she ever got yelled at by grandpa.
When I entered 5th grade, the first math class the teacher told us to get out some paper and a pencil. He then instructed us to write the answers only to our 2s through 12s times tables. He gave us five minutes.
We were to write out.
2 4 6 8 10 12 14 16 1 8 20 22 24
3 6 9 12 15 18 21 24 27 30 33 36
4 6 8…
12 24 36 48 60 72 84 96 108 120 132 144
We did this drill every day for a semester. He gave prizes to those who finished first. (I usually finished in the top three)
What amazed me is that some of the kids did not realize the all but one of the answers to the 12 times table were to be found at the bottom of the columns they had already written out. They weren’t getting that 2 x 12 = 12 x 2!
I can do a great deal of basic math in my head and I think most of the credit goes to this teacher. I can’t even remember his name.
OTOH I had to use word to spell check becasue I, for some strange reason, always want to spell answer as answere.
My very, very first job was working at a non-profit sno cone stand (shaved ice, to some of you) for $1/hour. We had no cash registers, just cash boxes, and I was taught to give change by counting up to what the person had given me. Worked every time! 
Oh man, that was me in college (I worked at a Hallmark cards store). :o
I clearly remember being confused as hell the first few times customers gave me more money than I’d asked them for…luckily, only one of them called me an idiot while I figured it out/they explained it to me. (I mostly had nice customers: great big meanies don’t tend to buy greeting cards. 
) I was completely mathematically challenged in those days: I’d been told all though high school that I was horrible in math, and the semester before I started working at Hallmark I had a Calc II teacher who believed that only Math majors belonged in his class (or if you weren’t a Math major, you should at least be a guy), so I believed the hype and the simplest things like that would throw me.
BUT, thanks to my sno cone stand experience, I was the only student employee who could still make change when the registers broke! 
When I was in grade school, my father – who has a B.S. in Math – attempted to motivate me by saying that if I could ever come to him and perfectly recite the times table up to and including twelve, he would give me $10. There was no expiration date on his offer. I am now 33 years old, and have never taken him up on it.
I have good basic maths skills, but I tend to think them out the long way - if I’m figuring out discounts, I’ll divide the number by 10, multiply it by whatever, then subtract that number from the total. It looks even more convoluted written down, but it works.
Have you seen the registers at some fast food joints where the buttons are labeled not with numbers, but with pictures of the items being sold?
They also (of course) add tax and calculate the change for the sale.
I believe some of the latest models dispense the correct amount of change in a little tray…