Give the step-by-step reasoning process and then the final answer. Each class uses 200 sheets of paper per day. The school uses a total of 9000 sheets of paper every week. If there are 5 days of school days, how many classes are there in the school?
Each class uses 200 x 5 = 1000 sheets of paper in a week. Thus, there are 9000 / 1000 = 9 classes in the school.
The final answer: 9.