Personally, I think it's all right here. This skill is called the second kill skill, as the name implies, because it is fast and accurate. I think the lecturer has his own thinking logic. He knows where these students are wrong, knows the core of the problem, and then hits the nail on the head with the most direct method, so he will occupy a great advantage in the exam and leave the rest of the time for the big questions behind to help each candidate get good grades. I hope my answer can solve your problem.