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Difference between revisions of "2019 AMC 12A Problems/Problem 13"

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<math>\textbf{(A)}\ 144\qquad\textbf{(B)}\ 216\qquad\textbf{(C)}\ 256\qquad\textbf{(D)}\ 384\qquad\textbf{(E)}\ 432</math>
 
<math>\textbf{(A)}\ 144\qquad\textbf{(B)}\ 216\qquad\textbf{(C)}\ 256\qquad\textbf{(D)}\ 384\qquad\textbf{(E)}\ 432</math>
  
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==Solution==
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==Solution 1==
  
 
The <math>5</math> and <math>7</math> can be painted with no restrictions because the set of integers does not contain a multiple or proper factor of <math>5</math> or <math>7</math>. There are 3 ways to paint each, giving us <math>\underline{9}</math> ways to paint both. The <math>2</math> is the most restrictive number. There are <math>\underline{3}</math> ways to paint <math>2</math>, but WLOG, let it be painted red. <math>4</math> cannot be the same color as <math>2</math> or <math>8</math>, so there are <math>\underline{2}</math> ways to paint <math>4</math>, which automatically determines the color for <math>8</math>. <math>6</math> cannot be painted red, so there are <math>\underline{2}</math> ways to paint <math>6</math>, but WLOG, let it be painted blue. There are <math>\underline{2}</math> choices for the color for <math>3</math>, which is either red or green in this case. Lastly, there are <math>\underline{2}</math> ways to choose the color for <math>9</math>.
 
The <math>5</math> and <math>7</math> can be painted with no restrictions because the set of integers does not contain a multiple or proper factor of <math>5</math> or <math>7</math>. There are 3 ways to paint each, giving us <math>\underline{9}</math> ways to paint both. The <math>2</math> is the most restrictive number. There are <math>\underline{3}</math> ways to paint <math>2</math>, but WLOG, let it be painted red. <math>4</math> cannot be the same color as <math>2</math> or <math>8</math>, so there are <math>\underline{2}</math> ways to paint <math>4</math>, which automatically determines the color for <math>8</math>. <math>6</math> cannot be painted red, so there are <math>\underline{2}</math> ways to paint <math>6</math>, but WLOG, let it be painted blue. There are <math>\underline{2}</math> choices for the color for <math>3</math>, which is either red or green in this case. Lastly, there are <math>\underline{2}</math> ways to choose the color for <math>9</math>.
  
 
<math>9 \cdot 3 \cdot 2 \cdot 2 \cdot 2 \cdot 2 = \boxed{\textbf{(E) }432}</math>.
 
<math>9 \cdot 3 \cdot 2 \cdot 2 \cdot 2 \cdot 2 = \boxed{\textbf{(E) }432}</math>.
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==Solution 2==
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We note that the primes can be colored any of the <math>3</math> colors since they don't have any proper divisors other than <math>1</math>, which is not in the list. Furthermore, <math>6</math> is the only number in the list that has <math>2</math> distinct prime factors (namely, <math>2</math> and <math>3</math>), thus we do casework on <math>6</math>.
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Case 1: <math>2</math> and <math>3</math> are the same colors
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In this case, we have <math>3</math> primes to choose the color for (<math>2</math>, <math>5</math>, and <math>7</math>). Afterwards, <math>4</math>, <math>6</math>, and <math>9</math> have two possible colors, which will determine the color of <math>8</math>. Thus, there are <math>3^3\cdot 2^3=216</math> possibilities here.
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Case 2: <math>2</math> and <math>3</math> are different colors
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In this case, we have <math>4</math> primes to color. WLOG, we'll color the <math>2</math> first, then the <math>3</math>. Then there are <math>3</math> color choices for <math>2,5,7</math>, and <math>2</math> color choices for <math>3</math>. This will determine the color of <math>6</math> as well. After that, we only need to choose the color for <math>4</math> and <math>9</math>, which each have <math>2</math> choices. Thus, there are <math>3^3\cdot 2^3=216</math> possibilities here as well.
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Adding up gives <math>216+216=\boxed{\textbf{(E) }432}</math>.
  
 
==See Also==
 
==See Also==

Revision as of 22:35, 10 February 2019

Problem

How many ways are there to paint each of the integers $2, 3, \dots, 9$ either red, green, or blue so that each number has a different color from each of its proper divisors?

$\textbf{(A)}\ 144\qquad\textbf{(B)}\ 216\qquad\textbf{(C)}\ 256\qquad\textbf{(D)}\ 384\qquad\textbf{(E)}\ 432$

Solution 1

The $5$ and $7$ can be painted with no restrictions because the set of integers does not contain a multiple or proper factor of $5$ or $7$. There are 3 ways to paint each, giving us $\underline{9}$ ways to paint both. The $2$ is the most restrictive number. There are $\underline{3}$ ways to paint $2$, but WLOG, let it be painted red. $4$ cannot be the same color as $2$ or $8$, so there are $\underline{2}$ ways to paint $4$, which automatically determines the color for $8$. $6$ cannot be painted red, so there are $\underline{2}$ ways to paint $6$, but WLOG, let it be painted blue. There are $\underline{2}$ choices for the color for $3$, which is either red or green in this case. Lastly, there are $\underline{2}$ ways to choose the color for $9$.

$9 \cdot 3 \cdot 2 \cdot 2 \cdot 2 \cdot 2 = \boxed{\textbf{(E) }432}$.

Solution 2

We note that the primes can be colored any of the $3$ colors since they don't have any proper divisors other than $1$, which is not in the list. Furthermore, $6$ is the only number in the list that has $2$ distinct prime factors (namely, $2$ and $3$), thus we do casework on $6$.

Case 1: $2$ and $3$ are the same colors

In this case, we have $3$ primes to choose the color for ($2$, $5$, and $7$). Afterwards, $4$, $6$, and $9$ have two possible colors, which will determine the color of $8$. Thus, there are $3^3\cdot 2^3=216$ possibilities here.

Case 2: $2$ and $3$ are different colors

In this case, we have $4$ primes to color. WLOG, we'll color the $2$ first, then the $3$. Then there are $3$ color choices for $2,5,7$, and $2$ color choices for $3$. This will determine the color of $6$ as well. After that, we only need to choose the color for $4$ and $9$, which each have $2$ choices. Thus, there are $3^3\cdot 2^3=216$ possibilities here as well.

Adding up gives $216+216=\boxed{\textbf{(E) }432}$.

See Also

2019 AMC 12A (Problems • Answer Key • Resources)
Preceded by
Problem 12 		Followed by
Problem 14
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25
All AMC 12 Problems and Solutions

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