The book is open exactly in the middle.
How many pages are there in the book if it is known the “Chapter next” page number is not a prime number?
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When a book is open, the even-numbered pages are on the left side, the odd-numbered pages – on the right side. As a result the two-digit fragment on the left can only end with the even 4 (the odd 1 and 7 can not be there). In contrast the revealed part of the right number belongs to the odd 3 (the even 8 can not be there).
There are 5 possibilities for the left-numbered page: 34, 54, 64, 84 or 94.
There are 4 sheets or 8 pages between the left-numbered page and the right-numbered page. This leads to 5 right-numbered page possibilities: 43, 63, 73, 93 or 103.
Being 2 pages less, this leads to the respective 5 possibilities for the “Chapter next” page: 41, 61, 71, 91, and 101. Among these numbers only 91 is not a prime number. Since it starts the second half of the book, the half of the book is exactly 90 pages. The entire book is 180 pages.