Each shelf of a bookcase contained 12 books. If the librarian took out 21 books and rearranged the remaining books so that all shelves but one contained 8 books and the last shelf contained 15 books, how many shelves does the bookcase have?	Choose the correct choice from the following options:
A) 9 B) 4 C) 5 D) 6 E) 7
Let x be the number of shelves.   No of books in the beginning =12*x   After taking out 21 books No of remaining books = 12x -21   when the remaining books are rearranged, each shelf except the last shelf has 8 books and the last shelf has 11 books. So No of remaining books is also equal to 8*(x-1)+11   12x-21=8(x-1)+15   4x=28   x=7. Answer 7
<result>E</result>
Each shelf of a bookcase contained 12 books. If the librarian took out 21 books and rearranged the remaining books so that all shelves but one contained 8 books and the last shelf contained 15 books, how many shelves does the bookcase have?