Originally Posted by County Mike
I feel less nerdy for not getting this one.
You didn't get it because you are intelligent.
The waiter is taking the picture to show that the idiot customer was too stupid/lazy to figure out that 23.00-16.20=6.80. Instead of figuring out how much the tip would be, he just wrote in "MATH", implying that the educational system failed him & he would just pass the buck (sorry, bad pun) off to someone else to figure it all out for him.