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
