Since 4 students read 11 books last month, on average 1 student reads 2.75 books per month. We can multiply 31 by 2.75 to find an estimation for the amount of books the whole class read. This is equal to 85.25.
So, the class reads 85.25 books per month.
We multiply this number by three to find the amount of books the students read in 3 months. 85.25*3=255.75 books.