Eric has been donating books he no longer needs to his local library. After 3 months of donating, he had 157 books left in his room. After 8 months of donating, he had 102 books left in his room. How many books per month did Eric donate on average between months 3 and 8?

Question

anonymous · Answer

An average (the mean) is
The Total Amount/Number of Amounts

anonymous · Answer

So you're going to want 
the amount of books he donated in that time period/the months he donated for