A recent census at a major university revealed that 60% of its students mainly used Macs. The rest primarily used PCs. At the time of the census, 67% of the school’s students were undergraduates. The rest were graduate students. In the census, 23% of respondents were graduate students who said that they Macs as their primary computers. Suppose we select a student at random from among those who were apart of the census and learn that the student mainly uses a Mac. Find the probability that this person is a graduate student.