This paper proposes modelling based learning as a tool for learning and teaching mathematics in high school. We report on an example of modelling real world problems in two high schools in Serbia where students were introduced for the first time to the basic concepts of modelling. Student use of computers and educational software, GeoGebra, was also part of this project. The students' task was to model a number of real world problems and situations. Students, having developed their model and tested their results, made a presentation to fellow students. In this situation, the teacher's role was changed from someone who gave out knowledge to someone who encouraged self learning and reasoning. This paper gives an example of a task which was given to students with a set of instructions. (Contains 4 figures.)