You just confirmed that they can earn 8000 yuan for just lecturing 20 hours. For a 60-hour course (3 credits), they can make 24000 yuan. For 2 60-hour courses, they double the earning to 48,000 yuan. Isn't this more than 10,000 yuan/month? In the US, it is quite normal for a professor to teach 2 courses in the same semester.