Confused...

I was a little confused today. We were discussing HW13, which for problem #2, asked us to find the average rate at which the oil slick grew. There seemed to be some confusion about how to do problem #2. Why? When this lesson was planned, Ms. Jones and I thought there would be very little confusion about #2. Were we mistaken to in thinking that?

To us, we thought of it as just another rate of change problem, which we have done alot of examples of -- even had several rate of change problems on our 2 quizzes. Why was this one different?