There are 100 books on a shelf. Of these, 40% are fiction, 30% are biographies, and the rest are science books. How many more science books are there than biographies?
Let's break down the problem step-by-step:

1. We know that there are 100 books on the shelf.
2. Of these, 40% are fiction, so there are (0.4 * 100) = 40 fiction books.
3. Similarly, 30% are biographies, so there are (0.3 * 100) = 30 biography books.
4. The remaining books are science books. Since we've accounted for 70 books already (40 fiction and 30 biographies), there must be 100 - 70 = 30 science books.
5. Now, we need to find how many more science books there are than biographies. So, we subtract the number of biographies from the number of science books: 30 - 30 = 0

There are no more science books than biographies.