A university plans on conducting a survey of its recent graduates to determine information on their yearly salaries. It randomly selected 200 recent graduates and sent them questionnaires dealing with their present jobs. Of these 200, however, only 86 questionnaires were returned. Suppose that the average of the yearly salaries reported was $75,000. a. Would the university be correct in thinking that $75,000 was a good approximation to the average salary level of all its graduates? Why? b. If your answer to Part a above is no, can you think of any set conditions relating to the group that returns questionnaires for which $75,000 would be a good approximation