answer: #Person1# and #Person2# eat many dumplings. #Person1# is trying to give #Person3# a tip but both #Person2# and #Person3# don't understand #Person2#'s word 'keep the change'.
documents: #Person1#: I'm absolutely stuffed. I don't think I've ever eaten so much at one sitting. 
  #Person2#: same here. How did we manage to eat seven jin of dumplings? 
  #Person1#: well, I had five of them. the bill, please. 
  #Person3#: that will be 71 yuan. 
  #Person2#: here's 80. keep the change. 
  #Person3#: I'm sorry, sir? 
  #Person2#: he means, 'Could we get the change please? ' 
  #Person3#: of course, one moment please. 
  #Person2#: what did you mean when you said 'keep the change'? 
  #Person1#: it's called a 'tip'. Think of it as a gift given to the server to show appreciation for their good service. It's common practice in the States.