One of my most enduring (and possibly endearing) characteristics is that I am hopeless at math. If I can’t do a calculation in my head and/or use my fingers, my brain shuts down. Ask me to add seven and six and I picture holding up seven fingers which means there are three left to ten, three = one half of six, so then plus those leftover three, must equal 13. This process takes about half as long to do as it did to read, which is way too long.
It’s pretty hard to go through life hindered this way, but I’ve developed lots of workarounds to compensate. In the US, I know that I can leave 20% on a bill by moving the decimal of the total to the left one digit and then doubling the number that’s created. Thus, a tab for $47.65 becomes $4.76; 4 x 2 = 8, but there’s the 76 to consider so I’ll just add another dollar, rounding the tip out to $9. That 9 + 48 (round up after .5 my teacher told me) = 57. I’m usually splitting the bill with someone (always going halfsies), so we’ll end up leaving between $57 and $60. And I can walk away feeling good about the transaction without having to do anything more strenuous than dividing 57 (which I have to do by adding 3.5 [half of 7] to 25 [half of 50] since even something so simple must be broken down into its corresponding pieces.)
When paying for something at the register, my sister long ago taught me a trick so that I could get “good change” back. This usually involved getting quarters instead of coins of lesser denominations or fives instead of singles. I’m bad at math, so I can’t explain how it works, but the principle is this: if something costs $18 and you want a five back instead of two ones, give $23.
In France, cashiers are very good at math and will ask you to make up the difference, sometimes with crazy amounts of coins just to avoid giving back a more insane amount. I usually preempt such requests by always using my sister’s trick when I can.
Recently, the checker didn’t see where I was going when I handed her 24,06€ on an 8,76€ total, but when she entered the numbers into the register and got my change back total, her eyebrow raised and she looked at me and nodded in respect.