Difference between revisions of "2002 AMC 8 Problems/Problem 17"

Latest revision as of 13:35, 29 October 2024

Problem

In a mathematics contest with ten problems, a student gains 5 points for a correct answer and loses 2 points for an incorrect answer. If Olivia answered every problem and her score was 29, how many correct answers did she have?

$\text{(A)}\ 5\qquad\text{(B)}\ 6\qquad\text{(C)}\ 7\qquad\text{(D)}\ 8\qquad\text{(E)}\ 9$

Solution 1

We can try to guess and check to find the answer. If she got five right, her score would be $(5*5)-(5*2)=15$ . If she got six right her score would be $(6*5)-(2*4)=22$ . That's close, but it's still not right! If she got 7 right, her score would be $(7*5)-(2*3)=29$ . Thus, our answer is $\boxed{\text{(C)}\ 7}$ . ~avamarora

Solution 2

We can start with the full score, 50, and subtract not only 2 points for each incorrect answer but also the 5 points we gave her credit for. This expression is equivalent to her score, 29. Let $x$ be the number of questions she answers correctly. Then, we will represent the number incorrect by $10-x$ .

$\begin{align*} 50-7(10-x)&=29\\ 50-70+7x&=29\\ 7x&=49\\ x&=\boxed{\text{(C)}\ 7} \end{align*}$

Solution 3

Suppose she got $x$ questions right. Then she got $10 - x$ questions wrong. Since she gains 5 points for a correct answer and loses 2 for an incorrect one, we can solve $5x - 2(10 - x) = 29$ to get that $x = \boxed{\text{(C)}\ 7}$ .

~cxsmi

Solution 4

We see that Olivia's score is odd. Since subtracting multiples of $2$ (even numbers) does not change a number's parity (odd or even), Olivia's score from only her correct answers must be odd. Then, we test odd multiples of $5$ greater than $29$ to see which one works. The smallest odd multiple of $5$ greater than $29$ is $35$ , meaning $7$ answers were correct and the remaining $3$ answers were wrong. We see that the final score is $7(5)-3(2)=35-6=29,$ which is the score Olivia got. Thus, the answer is $\boxed{(C) 7}$ .

Video Solution

https://youtu.be/8YXPMTjOyvM Soo, DRMS, NM

https://www.youtube.com/watch?v=aTeyOXo6-Uo ~David

Video Solution by OmegaLearn

https://youtu.be/rQUwNC0gqdg?t=1560

~pi_is_3.14

Video Solution by WhyMath

https://youtu.be/9dl4iKzW6Tg

@@ Line 21: / Line 21: @@
 Suppose she got <math>x</math> questions right. Then she got <math>10 - x</math> questions wrong. Since she gains 5 points for a correct answer and loses 2 for an incorrect one, we can solve <math>5x - 2(10 - x) = 29</math> to get that <math>x = \boxed{\text{(C)}\ 7}</math>.
-~ cxsmi
+~[https://artofproblemsolving.com/wiki/index.php/User:Cxsmi cxsmi]
+==Solution 4==
+We see that Olivia's score is odd. Since subtracting multiples of <math>2</math> (even numbers) does not change a number's parity (odd or even), Olivia's score from only her correct answers must be odd. Then, we test odd multiples of <math>5</math> greater than <math>29</math> to see which one works. The smallest odd multiple of <math>5</math> greater than <math>29</math> is <math>35</math>, meaning <math>7</math> answers were correct and the remaining <math>3</math> answers were wrong. We see that the final score is
+<cmath>7(5)-3(2)=35-6=29,</cmath>
+which is the score Olivia got. Thus, the answer is <math>\boxed{(C) 7}</math>.
 ==Video Solution==
@@ Line 34: / Line 39: @@
 ~pi_is_3.14
+==Video Solution by WhyMath==
+https://youtu.be/9dl4iKzW6Tg
 ==See Also==
 {{AMC8 box|year=2002|num-b=16|num-a=18}}
 {{MAA Notice}}

2002 AMC 8 (Problems • Answer Key • Resources)
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