All of the problems in the lesson also had ordered pairs consisting of positive integers for their solutions, so students were not required to think about how algebraic approaches to systems with noninteger solutions yield exact answers, whereas graphical and tabular approaches generally do not. (citejournal.org)
These have negative and fraction solutions - I do have other resource available for positive integers only. (tes.com)
The process of numbering shall simply go on ad infinitum as does the series of positive integers. (religion-online.org)