You are only considering first year students. How many are there? Of those how many live on campus?
Why is 30/100 wrong? Yes there are 30 1st year students who live on campus, but there are not 100 first year students.
Let me ask you this question. Suppose the 2nd row in the chart was not there at all. Could this problem be done? If yes, then what would the answer be? Please answer that question.
You want to divide by 100. Lets assume that instead of others it said 2-4 year students. But that is not complete. Why not add on graduate students, teaching assistants, faculty, deans ....? Because the sample space in this problem is just 1st year students.
The exact translation to the problem is find the probability that a student lives on campus given that they are a first-year student