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Difference between revisions of "1981 AHSME Problems/Problem 12"

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==Solution (Answer Choices)==
 
==Solution (Answer Choices)==
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Answer Choice <math>A</math>: It is obviously incorrect because if <math>M</math> is <math>50</math> and we increase by <math>50</math>% and then decrease <math>49</math>%, <math>M</math> will be around <math>37</math>.
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Answer Choice <math>B</math>: If <math>p</math> is <math>100</math> and <math>q</math> is <math>50</math>, it should be equal but instead we get <math>100</math> is more than <math>1</math>. This is therefore also incorrect.
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Answer Choice <math>C</math>: Obviously incorrect since if <math>q</math> is larger than <math>1</math>, this is always valid since <math>\frac {1}{1-q}</math> is less than <math>0</math>.
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Answer Choice <math>D</math>: If <math>p</math> is <math>100</math> and <math>q</math> is <math>50</math>, it should be equal but instead we get <math>100</math> is less than <math>\frac {5000}{150}</math>. Therefore, <math>D</math> is also incorrect.
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Answer Choice <math>E</math>: If <math>p</math> is <math>100</math> and <math>q</math> is <math>50</math>, it should be equal, and if we check our equation, we get <math>\frac {5000}{50}</math> = <math>100</math>. Therefore, our answer is <math>\boxed {(E)\dfrac{100q}{100-q}}</math>

Revision as of 18:02, 23 October 2021

Problem

If $p$, $q$, and $M$ are positive numbers and $q<100$, then the number obtained by increasing $M$ by $p\%$ and decreasing the result by $q\%$ exceeds $M$ if and only if

$\textbf{(A)}\ p>q \qquad\textbf{(B)}\ p>\dfrac{q}{100-q}\qquad\textbf{(C)}\ p>\dfrac{q}{1-q}\qquad \textbf{(D)}\ p>\dfrac{100q}{100+q}\qquad \\ \textbf{(E)}\ p>\dfrac{100q}{100-q}$

Solution (Answer Choices)

Answer Choice $A$: It is obviously incorrect because if $M$ is $50$ and we increase by $50$% and then decrease $49$%, $M$ will be around $37$.

Answer Choice $B$: If $p$ is $100$ and $q$ is $50$, it should be equal but instead we get $100$ is more than $1$. This is therefore also incorrect.

Answer Choice $C$: Obviously incorrect since if $q$ is larger than $1$, this is always valid since $\frac {1}{1-q}$ is less than $0$.

Answer Choice $D$: If $p$ is $100$ and $q$ is $50$, it should be equal but instead we get $100$ is less than $\frac {5000}{150}$. Therefore, $D$ is also incorrect.

Answer Choice $E$: If $p$ is $100$ and $q$ is $50$, it should be equal, and if we check our equation, we get $\frac {5000}{50}$ = $100$. Therefore, our answer is $\boxed {(E)\dfrac{100q}{100-q}}$

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