November 2019 LSAT
Section 4
Question 21

# All of the students at Harrison University live in one of two residence complexes, either Pulham or Westerville. Alth...

Replies

SamA on April 25, 2020

Hello @mg123,This is a classic example of LSAT math, and these questions always require a good understanding of rates and percentages. It is important that you are comfortable with these, because I can guarantee you will encounter more questions like this.

Let's break this question down. Harrison University students are divided into two groups: Pulham and Westerville. We are then given a statistic about Harrison students as a whole. 38% of them take night classes. We also know that 29% of Westerville students take night classes.

H: 38%

W: 29%

P: ???

Notice that the Westerville rate is well below the school's rate. How is it possible for the school's rate to reach 38% when Westerville is only 29%? Pulham would have to bring this percentage up considerably. These are the only two housing groups, so there is no other factor to consider.

Think of it this way. If Pulham students were exactly 38%, it would be impossible for the whole school's rate to be 38%, because Westerville would bring this number down. Therefore, Pulham has to be above the rate of the school as a whole, enough to make up for Westerville. This is why A is correct.

For the sake of easy math, let's assume that the housing groups each have 100 students.

H: 76/200 take night classes (38%)

W: 29/100 take night classes (29%)

What does Pulham's rate have to be in order for this to make sense?

76-29 = 47

P: 47/100 take night classes (47%)

The concept is the same, even if the two houses do not have an equal number of students.

Chase on June 5, 2020

Okay but that explanation assumes a certain amount of students living in each dorm that is never explicitly stated in the stimulus. For all we know, 29% of students in Westerville can account for 37% of total night class taking students.trevor on August 23, 2020

^! Would love an explanation on what Chase said pleaseruchitaj on August 25, 2020

^Same! I chose B instead of A.Gabe85 on September 6, 2020

I would also like an explanation regarding what Chase said, thanks!JessQ on December 10, 2020

An explanation regarding what Chase said please?Remi on April 26, 2021

hey guys just asked a tutor, here is the explanation ; see where you're going with that. Indeed, they never said how many live in each. However, there are only two places the students can live--P or W. If only 29% of the students in W meet the qualification, then you can think of it this way--even if you assumed every student lived in W, that is at most 29% of the total student population. You would need some students in P to be able to get to the average of 38%. If you had, say, a 90/10 split: 90% of your total student body is in W and 10% is in P, then, since W has 29% of night class students, you would still need a large percentage (over 38%) of P to be night students in order to have 38% of TOTAL students be night students. Vice Versa, if P were 90% of your total student body, and W were only 10% of your total student body, then the night students in W would be only a small fraction of the total night students. Since night students make up 38% of the total student body, then at least 38% must come from P in that scenario to be able to make 38% of the TOTAL student body night students.