Probability Question


New member
Nov 11, 2018
Hello all

I was wondering if someone could help explain the following probability question:-


The answer is 30/40 however my first attempt was 30 / 100 but i was told i was wrong.

Can anyone go through this question and explain why 30/100 is wrong and 30/40 is correct?

Thank you


Elite Member
Dec 30, 2014
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