The problem works for me. I don't see it in your Payer-Homework1 set. Do you have a different problem set in mind? Can you be more specific about the error message and where you have linked the problem? Also, for a problem in the OPL, it is best to give the full path name of the file, not the contents of the file so that we can locate it more easily.
PREP 2015 Question Authoring - Archived
Undefined Subroutine error in Library problems
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