Let be "n" the number of books that are not math books.
According to the information given in the exercise, each case has 20 books and there are 72 cases. Knowing this, you can calculate the total number of books:

You know that the 10% of this total were math books. Therefore, you can set up the following equation (remember that a percent can be written as a Decimal number by dividing it by 100):

Evaluating, you get:

The answer is: 144 books are not math books.