I am doing a short research paper on how to teach combinations. Here in Malaysia, we teach permutation and combination to students in Form 5 ( age of 17-18 ). Students always have problems with this topic. I wonder if students in other countries facing this problem? How teachers teach this topic effectively? Students always feel frustrated when they fail to get the correct answers. I would like to investigate if CAI can help by using the elaborative feedbacks to give extra explanation. Could someone please give me some comments how to teach this topic effectively? I greatly appreciate it.