Financial questions that need an answer

Ordering the entire amount printed at the beginning of the year ties up valuable storage space and capital. However, running off the copies in several partial runs throughout the year results in added costs for setting up apiece printing run. Setting up apiece production run costs $1000. The carrying costs, figured on the average number of books in storage, are $0.50 per book. Find the economic lot size, that is the production run size that minimizes the total setting up and carrying costs.
I had this question on a test and got it wrong. Can somebody explain how to do it?