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Difference between revisions of "2018 AMC 10A Problems/Problem 8"

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Joe has a collection of 23 coins, consisting of 5-cent coins, 10-cent coins, and 25-cent coins. He has 3 more 10-cent coins than 5-cent coins, and the total value of his collection is 320 cents. How many more 25-cent coins does Joe have than 5-cent coins?
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== Problem ==
 +
 
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Joe has a collection of <math>23</math> coins, consisting of <math>5</math>-cent coins, <math>10</math>-cent coins, and <math>25</math>-cent coins. He has <math>3</math> more <math>10</math>-cent coins than <math>5</math>-cent coins, and the total value of his collection is <math>320</math> cents. How many more <math>25</math>-cent coins does Joe have than <math>5</math>-cent coins?
  
 
<math>\textbf{(A) }  0  \qquad        \textbf{(B) }  1  \qquad    \textbf{(C) }  2  \qquad  \textbf{(D) }  3  \qquad  \textbf{(E) }  4 </math>
 
<math>\textbf{(A) }  0  \qquad        \textbf{(B) }  1  \qquad    \textbf{(C) }  2  \qquad  \textbf{(D) }  3  \qquad  \textbf{(E) }  4 </math>
  
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==Solution 1==
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==Solution 1 (One Variable)==
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Let <math>x</math> be the number of 5-cent stamps that Joe has. Therefore, he must have <math>(x+3)</math> 10-cent stamps and <math>(23-(x+3)-x)</math> 25-cent stamps. Since the total value of his collection is 320 cents, we can write
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Let <math>x</math> be the number of <math>5</math>-cent coins that Joe has. Therefore, he must have <math>(x+3) \ 10</math>-cent coins and <math>(23-(x+3)-x) \ 25</math>-cent coins. Since the total value of his collection is <math>320</math> cents, we can write
 +
<cmath>\begin{align*}
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5x + 10(x+3) + 25(23-(x+3)-x) &= 320 \\
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5x + 10x + 30 + 500 - 50x &= 320 \\
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35x &= 210 \\
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x &= 6.
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\end{align*}</cmath>
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Joe has six <math>5</math>-cent coins, nine <math>10</math>-cent coins, and eight <math>25</math>-cent coins. Thus, our answer is
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<math>8-6 = \boxed{\textbf{(C) } 2}.</math>
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 +
~Nivek
 +
 
 +
==Solution 2 (Two Variables)==
 +
Let the number of <math>5</math>-cent coins be <math>x,</math> the number of <math>10</math>-cent coins be <math>x+3,</math> and the number of <math>25</math>-cent coins be <math>y.</math>
  
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\begin{align*}
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Set up the following two equations with the information given in the problem:
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5x+10(x+3)+25(23-(x+3)-x)= 320 \\
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<cmath>5x+10(x+3)+25y=320 \Rightarrow 15x+25y+30=320 \Rightarrow 15x+25y=290</cmath>
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& \Rightarrow 5x+10(x+3)+25(20-2x)= 320 \\
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<cmath>x+x+3+y=23 \Rightarrow 2x+3+y=23 \Rightarrow 2x+y=20</cmath>
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& \Rightarrow 5x+10x+30+500-50x= 320 \\
 
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& \Rightarrow 35x= 210 \\
 
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& \Rightarrow x= 6 \
 
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\end{align*}
 
  
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Joe has 6 5-cent stamps, 9 10-cent stamps, and 8 25-cent stamps. Thus, our answer is
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From there, multiply the second equation by <math>25</math> to get <cmath>50x+25y=500.</cmath>
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<math>8-6=\boxed{2}</math>
 
  
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~Nivek
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Subtract the first equation from the multiplied second equation to get <math>35x=210,</math> or <math>x=6.</math>
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Substitute <math>6</math> in for <math>x</math> into one of the equations to get <math>y=8.</math>
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Finally, the answer is <math>8-6=\boxed{\textbf{(C) } 2}.</math>
 +
 
 +
- mutinykids
 +
 
 +
==Solution 3 (Three Variables)==
 +
Let <math>n,d,</math> and <math>q</math> be the numbers of <math>5</math>-cent coins, <math>10</math>-cent coins, and <math>25</math>-cent coins in Joe's collection, respectively. We are given that
 +
<cmath>\begin{align*}
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n+d+q&=23, &(1) \\
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5n+10d+25q&=320, &(2) \\
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d&=n+3. &(3)
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\end{align*}</cmath>
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Substituting <math>(3)</math> into each of <math>(1)</math> and <math>(2)</math> and then simplifying, we have
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<cmath>\begin{align*}
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2n+q&=20, \hspace{17.5mm} &(1\star) \\
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3n+5q&=58. &(2\star)
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\end{align*}</cmath>
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Subtracting <math>(2\star)</math> from <math>5\cdot(1\star)</math> gives <math>7n=42,</math> from which <math>n=6.</math> Substituting this into either <math>(1\star)</math> or <math>(2\star)</math> produces <math>q=8.</math>
 +
 
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Finally, the answer is <math>q-n=\boxed{\textbf{(C) } 2}.</math>
 +
 
 +
~MRENTHUSIASM
 +
 
 +
==Video Solution (HOW TO THINK CREATIVELY!)==
 +
https://youtu.be/zbcnOfDJmQI
  
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==Solution 2==
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~Education, the Study of Everything
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Let n be the number of 5 cent coins Joe has, d be the number of 10 cent coins, and q the number of 25 cent coins. We are solving for q - n.
 
  
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We know that the value of the coins add up to 320 cents.
 
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Thus, we have 5n + 10d + 25q = 320. Let this be (1).
 
  
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We know that there are 23 coins.
 
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Thus, we have n + d + q = 23. Let this be (2).
 
  
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We know that there are 3 more dimes than nickels, which also means that there are 3 less nickels than dimes.
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==Video Solutions==
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Thus, we have d - 3 = n.
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https://youtu.be/ZiZVIMmo260
  
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Plugging d-3 into the other two equations for n, (1) becomes 2d + q - 3 = 23 and (2) becomes 15d + 25q - 15 = 320. (1) then becomes 2d + q = 26, and (2) then becomes 15d + 25q = 335.
 
  
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Multiplying (1) by 25, we have 50d + 25q = 650 (or 25^2 + 25). Subtracting (2) from (1) gives us 35d = 315, which means d = 9.
+
https://youtu.be/BLTrtkVOZGE
  
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Plugging d into d - 3 = n, n = 6.
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~savannahsolver
  
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Plugging d and q into the (2) we had at the beginning of this problem, q = 8.
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==Video Solution by OmegaLearn==
 +
https://youtu.be/HISL2-N5NVg?t=1861
  
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Thus, the answer is 8 - 6 = 2.
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~pi_is_3.14
  
 
== See Also ==
 
== See Also ==
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{{AMC10 box|year=2018|ab=A|num-b=7|num-a=9}}
 
{{AMC10 box|year=2018|ab=A|num-b=7|num-a=9}}
 
{{MAA Notice}}
 
{{MAA Notice}}
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 +
[[Category:Introductory Algebra Problems]]

Latest revision as of 14:14, 3 July 2023

Problem

Joe has a collection of $23$ coins, consisting of $5$-cent coins, $10$-cent coins, and $25$-cent coins. He has $3$ more $10$-cent coins than $5$-cent coins, and the total value of his collection is $320$ cents. How many more $25$-cent coins does Joe have than $5$-cent coins?

$\textbf{(A) } 0 \qquad \textbf{(B) } 1 \qquad \textbf{(C) } 2 \qquad \textbf{(D) } 3 \qquad \textbf{(E) } 4$

Solution 1 (One Variable)

Let $x$ be the number of $5$-cent coins that Joe has. Therefore, he must have $(x+3) \ 10$-cent coins and $(23-(x+3)-x) \ 25$-cent coins. Since the total value of his collection is $320$ cents, we can write \begin{align*} 5x + 10(x+3) + 25(23-(x+3)-x) &= 320 \\ 5x + 10x + 30 + 500 - 50x &= 320 \\ 35x &= 210 \\ x &= 6. \end{align*} Joe has six $5$-cent coins, nine $10$-cent coins, and eight $25$-cent coins. Thus, our answer is $8-6 = \boxed{\textbf{(C) } 2}.$

~Nivek

Solution 2 (Two Variables)

Let the number of $5$-cent coins be $x,$ the number of $10$-cent coins be $x+3,$ and the number of $25$-cent coins be $y.$

Set up the following two equations with the information given in the problem: \[5x+10(x+3)+25y=320 \Rightarrow 15x+25y+30=320 \Rightarrow 15x+25y=290\] \[x+x+3+y=23 \Rightarrow 2x+3+y=23 \Rightarrow 2x+y=20\]

From there, multiply the second equation by $25$ to get \[50x+25y=500.\]

Subtract the first equation from the multiplied second equation to get $35x=210,$ or $x=6.$

Substitute $6$ in for $x$ into one of the equations to get $y=8.$

Finally, the answer is $8-6=\boxed{\textbf{(C) } 2}.$

- mutinykids

Solution 3 (Three Variables)

Let $n,d,$ and $q$ be the numbers of $5$-cent coins, $10$-cent coins, and $25$-cent coins in Joe's collection, respectively. We are given that \begin{align*} n+d+q&=23, &(1) \\ 5n+10d+25q&=320, &(2) \\ d&=n+3. &(3) \end{align*} Substituting $(3)$ into each of $(1)$ and $(2)$ and then simplifying, we have \begin{align*} 2n+q&=20, \hspace{17.5mm} &(1\star) \\ 3n+5q&=58. &(2\star) \end{align*} Subtracting $(2\star)$ from $5\cdot(1\star)$ gives $7n=42,$ from which $n=6.$ Substituting this into either $(1\star)$ or $(2\star)$ produces $q=8.$

Finally, the answer is $q-n=\boxed{\textbf{(C) } 2}.$

~MRENTHUSIASM

Video Solution (HOW TO THINK CREATIVELY!)

https://youtu.be/zbcnOfDJmQI

~Education, the Study of Everything


Video Solutions

https://youtu.be/ZiZVIMmo260


https://youtu.be/BLTrtkVOZGE

~savannahsolver

Video Solution by OmegaLearn

https://youtu.be/HISL2-N5NVg?t=1861

~pi_is_3.14

See Also

2018 AMC 10A (Problems • Answer Key • Resources)
Preceded by
Problem 7 		Followed by
Problem 9
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All AMC 10 Problems and Solutions

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