Thre are two of us here that came up with the correct answer to the OPs question as per his book. Ok so the answer was given anyway and it's only my integrity that you may judge. Either way your working/answer was incorrect.
The original question and your Q31 are different in more than just numbers. And your method/answer for your example is correct. Have a read and fully understand what they each say before doing the working. Then have a look at the subtleties as presented in my previous post. If you still don't understand speak to a ground school instructor.