Mean starbucks lady!

So I went tanning today to start getting a base coat going for my 'trip'. Don't need to fry and get all red. I'll be going as much as possible in the next 15 days. Probably every other day. Neways, I went today and decided to get a mocha frap from Barnes and Nobles afterwards. So I am in line and I give a chick my order and then get to the cashier and she says, 3.15. And I give her a 5 dollar bill and then asked her she said 15 cents and she says yes '1-5' so I say I have a quarter and give it to her. I said this and she was still digging for the change and i say here's the quarter and she looks at me annoyed and says I don't know what to give you back and she holds all this change out to me and I'm like you said 3.15 right? So I get 2.10 back please and she gets all frustrated and slams her resigter and says to the another girl, call the manager in the harshes tone! I'm like what is this girl's problem. She tells me to move to the side and gets the next customer's order. The manager comes and asks what is wrong and the cashier tells him that I gave her a 5 and then she gets my change and closes the register and that is when I hand her the quarter. I was like what?!?! She is a lying piece of crap! I tell him um, no I gave her the quarter right after the 5 and she couldn't do the math and figure out how to give me 2.10 back. She had the register open when I gave her the money. He handed me the 2.10 and apolgized. I then said that she didn't have to be soo rude and grabbed my drink and walked off. I was soo upset! She didn't have to be soo mean and I don't understand why! Because she couldn't do that math or b/c she is just plain rude! It ruined my whole zen feeling after my tan too. Oh well, I'll get over it. Just upset that someone could be soo rude like that to customers.