###Human: 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?, ### Assistant: Each class uses 200 x 5 = 1000 sheets of paper in a week.
Thus, there are 9000/1000 = 9 classes in the school. 
 So the answer is :  9