What the hell are our public schools doing?

I just went to the grocery store to pick up a couple of things.
The total was $9.23. I had some cash in my pocket so that's how I intended to pay.
I peeled off a $20 and saw that I also had a quarter so I gave it to the woman at the register. The look of panic on her face was immediate.
The assistant manager saw it too and stepped over quickly to help. He looks at her register screen and says "we'll have to do it this way".
He pulls out his phone and starts doing something with it, then tells her to give me back a wrong amount that was too much.
I said "that's not right" and another manager comes over and starts pushing buttons on the register and comes up with a different wrong amount.
I told them, "you owe me $11.02".
By then a teenage girl cashier and a bag boy had stepped over and both had their phones out calculating.
I said out loud to all of them " A fifth grader should be able to figure this in his head in 2 seconds, we've got to do better". It's true too.
Of course, they all looked mad, and the cashier handed me my money.
I don't care though. We all need to start saying something in these situations. We pay taxes for these results.