These types of questions are nice because they allow you (1) to check your work (2) work towards the answer. On the other hand, I feel like these questions make you think less and make you less cautious about your moves because "you know where to go." Furthermore, I had this multiple times in exams where the "supposed" answer was something like n^(-1) and I was so damned sure I did everything correct and my answer was n^(-1/2). I literally wrote on my paper:

"I know that the answer is n^(-1) but I did all the steps and checked it multiple times and did not get a different answer than n^(-1/2). Sorry."

Of course, I was right and it was a small typo. A good friend of mine /crossed out/ his answer because /he thought it was wrong/ (he had my answer).

Luckily the question was scrapped and I got bonus points, but my point stands. My friend wasted a lot of time staring at a wrong answer -- giving him less time to think about the other questions!