Keats Library purchases a number of new books, all in the category of biography, and the library does not acquire any other books. With the addition of the new biographies, the biography collection of the library amounts to 55% of the total number of books in the library. If prior to the purchase, only 20% of the books in Keats Library were biographies, by what percent has the number of biographies in the library increased? Pick one
A) 270%  B) 310%  C) 350%  D) 390%  E) 430%
Let x be the number of new biographies added to the library.   Let b be the original number of biographies, so the original number of books was 5b.   0.55(5b+x) = b+x   1.75b = 0.45x   x = 3.9b   The answer is 390%.
<result>D</result>
Keats Library purchases a number of new books, all in the category of biography, and the library does not acquire any other books. With the addition of the new biographies, the biography collection of the library amounts to 55% of the total number of books in the library. If prior to the purchase, only 20% of the books in Keats Library were biographies, by what percent has the number of biographies in the library increased?