I recently had a curious experience while making a small purchase at one of those inappropriately-named "convenience" stores that litter our once-lovely landscape, simultaneously abusing our sensibilities and pandering to a host of perceived "needs" that no one really needs. I gave the cashier a bill that was larger than required, then realized that I could limit the amount of spare change in my pocket by adding a few coins to the amount I'd already given to her.
Unfortunately, I handed them over after the cashier had already started entering the amount on her cash register. Sensing her confusion, I explained why I was giving her more money, and told her the amount I should get back.
She froze for a moment, utterly pole-axed, then walked over to a different register, and entered the purchase and tendered amounts there. Once she learned that there had been no flim-flammery, no attempt, on my part, to flummox her with larcenous slight-of-hand, she returned to her original register -- where she began the transaction anew, from the beginning. All in all, the process consumed perhaps five or six minutes for each dollar of my purchase.
Think of the savings of time our society could amass if we were all required to master second-grade arithmetic.