In the 6th grade, he read = 15 books
In the 7th grade, he read = 21 books
The difference between the books he read = 21-15 =6 books
Total number of books from 6th to 7th grade = 15+21 = 36 books
So, the percentage of change in the number of books
Percentage in the number of books Miguel read from 6th to 7th grade? is 16.66%