At what rate is revenue rising or falling.

Problem:

"For the past few months, OHaganBooks.com has seen its weekly sales increase. At the moment OHaganBooks.com is selling 1000 books per week and its sales are rising at a rate of 200 books per week. Also, it is now selling all its book for $20 each, but the price is dropping at a rate of $1 per week. I need to know at what rate OHaganBooks.com's revenue is rising or falling given these conditions. I would also like to see that company's revenue increase at a rate of $5,000 per week. At what rate would sales have to have been increasing to accomplish this?"

So those are the two questions I have to answer. Pretty simple I'm guessing. I understand (R=pq) which is not hard to calculate for each week. I put the answers out in a table showing the difference in revenue from the previous week, and the total revenue for the week.

However, I'm guessing there is some calculus technique I need to use here and I'm not quite sure what it is. If anyone could take a look at it and see what they think I would greatly appreciate it.