The table above shows the number of students in a certain high school class who are boys and the number of students in

This topic has expert replies
Moderator
Posts: 1613
Joined: 29 Oct 2017
Thanked: 1 times
Followed by:5 members

Timer

00:00

Your Answer

A

B

C

D

E

Global Stats

2015-10-26_2106.png
The table above shows the number of students in a certain high school class who are boys and the number of students in the class who are studying biology. What is the total number of students in the class?

(1) Of the boys in the class, 15 are studying biology.
(2) The number of girls in the class is twice the number of students in the class who are NOT studying biology.

Answer: B

Source: Official Guide

GMAT/MBA Expert

User avatar
GMAT Instructor
Posts: 15476
Joined: 08 Dec 2008
Location: Vancouver, BC
Thanked: 5254 times
Followed by:1266 members
GMAT Score:770

Timer

00:00

Your Answer

A

B

C

D

E

Global Stats

M7MBA wrote:
Sun Apr 11, 2021 12:24 pm
2015-10-26_2106.png

The table above shows the number of students in a certain high school class who are boys and the number of students in the class who are studying biology. What is the total number of students in the class?

(1) Of the boys in the class, 15 are studying biology.
(2) The number of girls in the class is twice the number of students in the class who are NOT studying biology.

Answer: B

Source: Official Guide
ASIDE: Since the diagram closely resembles a Double Matrix Method diagram, I'll treat it as such.

Target question: What is the total number of students in the class?

Statement 1: Of the boys in the class, 15 are studying biology.
Add this information to our diagram to get:
Image

Since the top row must add to 18, we know that the top-right box must have 3 students.
Also, since the left-hand column must add to 26, we know that the bottom-left box must have 11 students.
Add this information to the diagram to get:
Image

At this point, we still don't have a value for the bottom-right box, which means that are many possible scenarios. Here are two possible scenarios:
Image
In the left-side diagram, there is a total of 49 students
In the right-side diagram, there is a total of 129 students

Since we cannot answer the target question with certainty, statement 1 is NOT SUFFICIENT

Statement 2: The number of girls in the class is twice the number of students in the class who are NOT studying biology.
We can add this information to our diagram as follows:
Image

KEY CONCEPT #1: the sum of the rows must equal the sum of the columns.
So, we can write: 18 + 2x = 26 + x
Solve to get: x = 8

KEY CONCEPT #2: the total number of students = the sum of the two rows
So, the total number of students = 26 + 8 = 34
The answer to the target question is there are 34 students in the class
Since we can answer the target question with certainty, statement 2 is SUFFICIENT

Answer: B

Cheers,
Brent
Image

A focused approach to GMAT mastery