Let us assume the number of books with which Jill started = x Number of books that Jill sold first is half and then Jill again bought 16 books, making her total number of books to 36. Then (x/2) + 16 = 36 (x/2) = 36 - 16 (x/2) = 20 x = 20 * 2 = 40
From the above deduction, it can be easily concluded that the number of books that Jill started with was 40. I hope that the procedure is clear enough for you to understand.