How many school text book that i can buy with 5 billion? (this is my civics homework)

In order to calculate an accurate answer, we would really need to know the cost
of the textbooks.  We don't know that, and they're not even all the same.

There's a good reason that you were given this question is Civics class, and
before I work on it for you, I want you to promise that you'll go to your teacher
on your way out of class someday soon, and tell your teacher that the guy who
helped you answer this question knows how the teacher feels, and that guy
feels the same way.

Ok.  In order to give you a feeling for the answer, let's try to come up with a
cost that might be a reasonably close figure to use for text books in general.
Now, I know that things have gone completely out of sight since I was in school,
so I'm going to try hard to go high with my numbers.  Let's say that the smallest
textbook costs $20, and the biggest one costs $60, and let's use  $40 as an
estimate for the average cost of every new textbook.

If that's true, then the number of text books that I could buy with $5 billion
would be

(5,000,000,000) / (40) = 125 million textbooks ! ! !

If the federal government would spend $5 billion on textbooks, there
would hardly be a high school student anywhere in the USA who didn't
have at least one brand new, up-to-date textbook.

And I can promise you that the book publishers would love it too. 

What do you mean determine? answer in comment section.