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. Determine how many books per month Eric had donated on average between months 3 and 8.

To find the average number of books donated per month, we first get the total number of books that Eric has donated within the span of (8 - 3) = 5 months. Since from 157 books, there were only 102 books left after 5 months, that means he has donated (157 - 102) = 55 books. That means that he has donated 55 books within the span of 5 monhs. Thus, the average number of books would be equal to (number of books) / (number of months) = 55 / 5 = 11 books per month. Answer: 11 books