The diversity of students in elementary school poses a challenge to the teaching and learning of mathematics. Differentiation is discussed as a useful approach to dealing with a heterogeneous group of students in a class. In this paper, we report on a part of a study that investigates a concept of differentiation for mathematics education in heterogeneous classes. This concept is based on open-ended tasks that can be solved on different levels of difficulty and therefore allow all students to work on the same tasks. For our study, we developed the open-ended task "Inventing Equations and Equation Sets" (IE-IES) and investigated the solution of 153 third graders with different achievement levels in mathematics. The exploratory cluster analysis reveals three groups that differed in terms of processing levels. Furthermore, data analysis suggests that students’ achievement level influences the solution.