answer: #Person1# and Mary are buying books and they find the paperback books are much cheaper.
documents: #Person1#: Mary, come here. I've found the book our teacher recommended us to buy last week.
  #Person2#: Really? How much is it? Oh, it's 42 yuan! How expensive!
  #Person1#: You know, it is the best seller for this year. It has had its fifth impression since its publication early this year.
  #Person2#: How come? I have never seen such a popular book before.
  #Person1#: Wait a minute. I found there were two printings. The paperback books must be much cheaper.
  #Person2#: You are right. Let's buy the cheaper one.