Planning teaching an unfamiliar mathematics problem: The role of teachers' experience in solving the problem and watching pupils solving it

During individual interviews, secondary school mathematics teachers were asked to plan teaching an unfamiliar and challenging mathematics problem. Each teacher watched a special video-clip showing a pair of pupils successfully coping with the task. Teachers' planning was analyzed at three stages: before they solved the problem, after solving it, and after they watched the video-clip. Solving the problem and observing the video had different effects on different planning components. After solving the problem the teachers generally modified their positioning of the task in the curriculum, as well as their expectations regarding pupils' ways of solving it and their difficulties. After watching the film, the teachers talked differently about the targets that may be achieved by means of this problem, and about the management of learning and the mathematical challenge they planned for their pupils; they also changed their expectations about pupils' ability to cope with the task.