Grade four and five is 3:5, and grade five is 75, so 75 books account for 5/8, so there are 75 books in grade four and five divided by 5/8, which is equal to 12 books. Similarly, the sixth grade accounts for 1/5 of the total, so the fourth and fifth grades account for 4/5 of the total, so the total number of books in this batch is.