The new batch of books in the school are distributed to the fourth, fifth and sixth grades according to the ratio of 3:4:5. There are 30 fewer books in grade four than in grade six. How many books are there in each grade? The fourth grade is 30 less than the sixth grade, that is, 3 volumes are 30 less than 5 volumes. Each copy is 30/(5-3) = 15 (book)

Grade 4: 15 * 3 = 45 (Ben)

Grade 5: 15 * 4 = 60 (Ben)

Grade 6: 15 * 5 = 75 (Ben)