I have trained a lot of teenagers (and my own sons) and it is amazing to me that they do not learn this in school. I began tutoring math all through my boys school adventures. The teenagers that I taught were shocked how easy and accurate it is to count money back that way.
On a side note: I was at the gym the other day and bought a smoothie. The price was $5.94 and I gave the girl $6.00 and because the cash register did not tell her how much change to give me, she did not know. It was 6 cents, I was horrified. I noticed 2 days later and she no longer worked there. So sad.