This article attempts to identify the kind of arguments that lower secondary school-level pupils use when they apply programming to solve a mathematical problem. By analyzing the data from one group of three pupils working together with Processing (Processing is a Java based tool primarily to learn programme visual effects supported and distributed by The Processing Foundation, https://processing.org/), the aim is to contribute to research and conceptualization surrounding how programming can be integrated in mathematics. The analysis provides examples of pupils' different arguments when they use Processing to solve a mathematical problem. An important finding is that the pupils revealed a clear progression in the argumentation categories, where they went from arguing very fundamentally to presenting more elaborated arguments. On the other hand, the trial-and-error method often used by pupils may also have had a negative impact on their mathematical arguments.