Within the field of Mobile Computer-Supported Collaborative Learning (mCSCL), we are interested in exploring the space of collaborative activities that enable students to practice communication, negotiation and decision-making skills. Collaboration is via learning activities that circumvent the constraints of fixed seating or locations of students. This paper presents one such collaborative learning activity that involves young students forming groups to achieve a specific goal (get the fractions on their handhelds to sum to one). Collaborative scaffolding is provided by the designed mobile collaborative application as well as by the students' social relationships and the teacher's facilitation. We report on our initial trials which show that the socio-technical design of the activity helps students in identifying their own strategies and stimulates collaboration. Beyond this specific application, we propose a generic model for mobile computer supported collaborative activities that can support a range of other tasks in learning languages, science or other disciplines. (Contains 20 figures.)