Difference between revisions of "2015 AMC 8 Problems/Problem 15"

Revision as of 11:05, 10 July 2021

Problem

At Euler Middle School, $198$ students voted on two issues in a school referendum with the following results: $149$ voted in favor of the first issue and $119$ voted in favor of the second issue. If there were exactly $29$ students who voted against both issues, how many students voted in favor of both issues?

$\textbf{(A) }49\qquad\textbf{(B) }70\qquad\textbf{(C) }79\qquad\textbf{(D) }99\qquad \textbf{(E) }149$

Solutions

Solution 1

We can see that this is a Venn Diagram Problem.

First, we analyze the information given. There are $198$ students. Let's use A as the first issue and B as the second issue.

$149$ students were for the A, and $119$ students were for B. There were also $29$ students against both A and B.

Solving this without a Venn Diagram, we subtract $29$ away from the total, $198$ . Out of the remaining $169$ , we have $149$ people for A and

$119$ people for B. We add this up to get $268$ . Since that is more than what we need, we subtract $169$ from $268$ to get

$\boxed{\textbf{(D)}~99}$ .

$[asy] defaultpen(linewidth(0.7)); draw(Circle(origin, 5)); draw(Circle((5,0), 5)); label("$A$", (0,5), N); label("$B$", (5,5), N); label("$99$", (2.5, -0.5), N); label("$50$", (-2.5,-0.5), N); label("$20$", (7.5, -0.5), N); [/asy]$

 (to editors: this looks really weird)Venn Diagram (I couldn't make circles),
                                              We need to know how many voted in favor for both

                                Issue A               Against both issues        Issue B
                               149 students                29 students          119 students

                                                         149+29+119=297
                                               297-198=99 students in favor for both

Solution 2

There are $198$ people. We know that $29$ people voted against both the first issue and the second issue. That leaves us with $169$ people that voted for at least one of them. If $119$ people voted for both of them, then that would leave $20$ people out of the vote, because $149$ is less than $169$ people. $169-149$ is $20$ , so to make it even, we have to take $20$ away from the $119$ people, which leaves us with $\boxed{\textbf{(D)}~99}$

Solution 3

Divide the students into four categories:

A. Students who voted in favor of both issues.
B. Students who voted against both issues.
C. Students who voted in favor of the first issue, and against the second issue.
D. Students who voted in favor of the second issue, and against the first issue.

We are given that:

$A + B + C + D = 198.$
$B = 29.$
$A + C = 149$ students voted in favor of the first issue.
$A + D = 119$ students voted in favor of the second issue.

We can quickly find that:

$198 - 119 = 79$ students voted against the second issue.
$198 - 149 = 49$ students voted against the first issue.
$B + C = 79, B + D = 49, \text{so} C = 50, D = 20, A = 99.$

The answer is $\boxed{\textbf{(D)}~99}$ .

Video Solution

https://youtu.be/OOdK-nOzaII?t=827

2015 AMC 8 (Problems • Answer Key • Resources)
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