The process of choosing strategies within mathematization is described in a multi-method study on solving additions and subtractions. The sample consisted of 40 dyads of 3rd-graders and their teacher from ten mainstream primary schools. From each classroom the two students rated as highest and the two rated as lowest were selected to take part in a session in which teacher and students worked together. Thus two research conditions were formed: high rating versus low rating. The teacher was not informed about the selection criterion for the students, and the students were not informed why they were invited to sit with the teacher and do sums. The task involved solving four sums, and it was video taped and analysed in three steps. In the first step, the conditions were compared quantitatively with respect to teacher's social support, instruction quality, and regulation, and students' response to social support. The second step involved identification of discourse patterns. The third step implied identification of student scripts following open-ended questions by teachers. The implications of the results for teacher training with respect to mathematics instruction are discussed. (Contains 2 tables and 4 figures.)