Find the total number of students in the class!

How many total people failed their driving test?

If you are given that a student owns an MP3 Player, what is the probability that the student owns a smart phone?

If you are given that a student owns a smart phone, what is the probability that the student also owns an MP3 player?

Which is more likely, that an underclassman planning to go to college is also planning to move out, or that an underclassman planning to move out is also planning to go to college? Justify your response with conditional probabilities.