Basic Math

I remembered something I wanted to put on here as I was walking the dog this morning. In the back of my mind, I have always kind of wanted to become a math teacher. I majored in Mathematics with an insurance minor. I think that I would be pretty effective, but not something I have made myself pursue yet. Yesterday morning, I was at Starbucks. This is my indulgence for the day, even though all I get is black coffee, I still enjoy starting my day with a cup. Well, yesterday morning, I had nothing at all to eat at my place, so I also purchased a fruit cup at Starbucks for more than it should have been, but hey, I was hungry! Total came to $5.67. As the total was coming up, I had a $20 bill out. I realized I had a $1 in my wallet as well, so, I hand her both bills. She took one in each hand and looks at me as if I just came out of a space ship. "So, what did you want back" she asks. At this point, I could have been an ass and said "my change", but decided an explanation of "$15, so I do not have so many $1's". I truly do not think she got it until she entered the amount in her trustee machine that she truly understood what I was attempting to do. Why does this situation make me want to pursue teaching? I would be that math teacher that would not allow you to use your calculator on the tests until the last 10 minutes of class and I would also require work to be shown for full credit.