As one of the difficult knowledge in senior high school mathematics, permutation and combination has its unique problem-solving thinking. The key is to abstract practical problems into mathematical problems in mathematical modeling. The flexibility of answering questions is often the direct reason for the difficulty in solving problems. Mastering the basic principles of permutation and combination and solving problems without attention or omission can often bring remarkable results to the accuracy of solving problems.

Common problem-solving methods include binding method, interpolation method, priority method, division method and so on.