The aim of this study is to find out if students reason too much during math lessons which leads to less time for practicing their basic knowledge in mathematics. When I collected the material, I chose to observe and interview at four different schools. When analyzing the material George Pólya's (1970) model for problem solving and Johan Litnher's (2008) framework for creative and imitative reasoning is used. The questions of this study is:

1. Are the students given space to exercise their basic knowledge and use their reasoning skills under the problem solving lessons?

2. Are there any tools that students can use based on their basic knowledge when working with problem solving?

3. What do different teachers consider regarding the use of reasoning and basic knowledge in a problem solving lesson?

The results show that students can both reason and learn the mathematical basic knowledge in the lessons with help of tools and challenging tasks. The results also show that teachers have different ways of looking at reasoning. Two teachers think that it helps to improve basic knowledge by reasoning. One teacher would like to use reasoning, but the students are too immature to do it by themselves, so he only use reasoning methods when they are in full class. The last teacher do not think that you can reason if you do not have the mathematical basic knowledge.