A learner clicks "Understood" button when he or she could understand a learning material or clicks "Not Understood" button when he or she could not understand. When the "Not Understood" button is clicked, the learning server machine transmits an explanation choice information to display a Web page including a knowledge tree that shows a structure of the learning material. The Web page contains headings of explanations and choosing ratios as end-nodes of the knowledge tree. The explanation describes the learning material in detail. The choosing ratio is a ratio of each of the explanations chosen by learners who attended the same lecture using the same learning material.

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