This paper examines the way two 10th graders cope with a non-standard generalisation problem that involves elementary concepts of number theory (more specifically linear Diophantine equations) in the geometrical context of a rectangle's area. Emphasis is given on how the students' past experience of problem solving (expressed through interplay among different modes of thinking and actions that show executive control and decision-making skills) supported them in their route towards generalisation. (Contains 4 figures and 1 table.)