Previous research has highlighted the importance of social relationships in mathematical group work while working on modelling activities. This study analyses the interaction of sixth-grade students in Primary Education (11 to 12 years old) carrying out a modelling task in groups with a Fermi problem used as the modelling activity. The focus of the study was to explore how students develop a mathematical model to solve a Fermi problem in groups. The data collected mainly came from the group discussions, although the students' productions were also considered. The results show that a variety of factors can influence group work and that model development is based on one student introducing an initial model and then, through social interaction with the other group members, the model is improved to develop a solid strategy that may be useful for solving the problem at hand.