Problems used in the open-approach method are non-routine problems. Furthermore, based on the openness described above, it is reasonable to classify the problems into three types: "Process is open," " End products are open" and "Ways to develop are open. " Several researchers use these names. The types are described below with typical examples. Process is open. This type of problem have multiple correct ways of solving the original problem. Needless to say, all mathematical problems are inherently open in this sense. However, the issue is that many school problems require only the answers or do not emphasize the process aspect of the problems. It is therefore important to verbalize that the process is open and ask for teachers to look at the problems at hand from such a viewpoint. The "card problem" below is one example of this type.