Dumb or Dumber?

     One evening I needed to take my son to the store for a 'scientific calculator'.  This isn't your couple dollar calculator either.  What the hell?  I don't care that it has been a few years since I was his age, I didn't even know what it was until that day.  I took him to the store and it came out to be $30.23... so I gave the cashier a $50 bill and $0.23.  She returned a $10 bill in my hand.  I sat there for a moment and was making sure the bloody mary wasn't still in effect from hours earlier.  I mentioned to the cashier.. "You owe me $20, not $10".  She replied rudely, "No, I owe you only $10".  So I sat and did the math with her... "$50 take away $30 is $20 left as change, not $10.  You owe me $20."  My son added, "My mom is a whiz at math, so you better listen to her".  I had to chuckle at that one.  She picked up her stupid little phone hanging behind her and called the manager.  At this point I opened the damn package that contained the calculator and showed the manager how I came to the answer of $20.  "I just want my $20", why is it that hard?"  I didn't think it was hard math to do.  Twenty five minutes later, and a $20 bill in my hand, I was finally on my way home.  Once I got home, I immediately went to my 10 year old and asked her, "What is the answer to 50-30?"... her response.... "20, duh mom".  To this day she still tells everyone that she had to help her mom with math.