The paper focuses on the analysis of mathematical argumentation of grade 7 students that participated in a joint mathematics-information technology project that had the form of an open problem. We combined the tools of Toulmin’s model –by measuring the density of the arguments in the basic argument elements- and that of argumentation schemes in order to trace any change across the course in these two parameters. As the results showed, the inquiry based approach of the project in conjunction with the open discourse environment of the course, resulted in a significant change in students’ argumentation. Their arguments became denser and more abstract in the sense of the schemes employed.