I consider the question in this post to be a conceptual question. This is because OP didn't understand which time interval to calculate - the question wasn't about the calculation itself.
Additionally I provided a hint which I think is in accordance with the current policy. Isn't it?
Although I do admit that after that, I said that I will provide a complete answer to the problem if OP doesn't manage to solve it themselves. At the time of posting the answer, I had not read the following:
If someone posts an answer to a homework-type question that gives away a complete or near-complete solution, in most cases it will be temporarily deleted.
I have since deleted that part of my answer. My apologies, I'm quite new to the site :)