Think that we have M professors and N coursesevery professor can wrote question for at least one course exam.we want to choose minimum number of professors in order to design question for all N courses exams.I try to hard but I only could model this question in a bipartite graph that one sections are  professors and the other sections includes courses every professors are connected to every courses that they can design question for them.we wish to choose minimum professors to cover all courses.one easy method is to check all cases and pick the minimum one in exponentioal time order...could any one help me to solve it in polynomial time?thanks
choose minimum number of M professors in polynomial time in order to design all N course exams
algorithms;complexity theory;graphs