question: This article reports the argumentation work of a group of trainee mathematics teachers in an experiment carried out in a virtual class (due to the emergence of COVID-19) during 2020. They worked with a task on fractions in an online assessment system with questions with random parameters and infinite possible correct answers. This was followed by a discussion of the strategies and the justifications, arguments and validations of these strategies and other conjectures that emerged. This article analyzes this work from a qualitative approach using the Mathematical Working Space as a theoretical framework. The results show that the discussion work led the trainee teachers to find interpretations for the algorithms processed by the computer, enhancing epistemic discourse and argumentation in the context of the use of technological artifacts. In turn, the same discourses allowed the future teachers to instrumentalize the processes to be used in new tasks.