Previously it accepted "sheets of paper" but apparently not on this question.
장 is translated to chapter but is also a counter for pieces of paper, I presume?
I would surmise that different contributors were responsible for different sections of the course, which unfortunately has led to some inconsistency. Part of the purpose of the "Beta" period is for users to flag / report issues such as this. Simply click on "report" and check "my answer should have been accepted." I wish they would provide a way to make suggestions that don't fit the flag report format, that would provide a chance for constructive feedback, but so far I am unaware of that possibility. I invite folks to post on https://www.duolingo.com/comment/24887752