The best-selling college statistics text, the thrill of statistics, sells 5 million copies every fall. some users keep the book, and some sell it back to the bookstore. suppose that 90% of all students who buy a new book sell it back, 80% of all students who buy a once-used book sell it back, and 60% of all students who buy a twice-used book sell it back. if a book has been used four or more times, the cover falls off, and it cannot be sold back.