Explain the title of the book. That is, what is devil's arithmetic?

http://www.gradesaver.com/the-devils-arithmetic/q-and-a/what-is-the-devils-arithmetic-83631

The specific meaning for the Devil's Arithmetic is that it refers to the the idea that for every one person who dies instead of you is one more day that you life and one less chance you will be Chosen for the gas chamber. It is an addition problem designed by evil or the devil.