What are two ways that the term "the devil's arithmetic" is used in the book?

I would say that the concept of the "devil's arithmetic"
is used in two specific ways in Yolen's work.  One way in which the concept is used is
to explain how survival is possible in the camp.  For those who lived in the camp, there
was an awful and incomprehensible formula which determine who would live and who would
die. In a struggle to find meaning, "the devil's arithmetic" is a means to help explain
issues of life and death in the camp.  From a philosophical standpoint, it was a way to
bring order into an existence that lacked it and defied logic and reasoning.  Similar to
this, the "logic" of the method was used to determine a code of conduct on how to live
in the camp.  We can see that "the devil's arithmetic" served two functions.  The first
was to explain how death was decided and the second was how one should live.  Within
this discussion is the larger issue of how structure and order was determined by the
devil and not God.  In this light, those who were in the camp, struggling to find
meaning, recognized that God was absent and in its place, there had to be some order,
which became "the devil's arithmetic."  In this configuration, there is both a statement
on faith and consciousness, with both overlapping one another.