She averages 16 pages every 30 minutes. If you take 336 and divide it by 16, you get 21 hours. But since she is reading 16 pages every half hour, that wouldn be correct. If it is asking how many hours it will take her to read the book, you have to take 16 and multiply it by 2. Then you get 32. Then, do the equation again. 336 divided by 32 is 10.5. So, if she reads the entire book in one go, it will take her 10 and a half hours to read the entire book, or 10.5.
at 
. (See definition of derivative)