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

(Created page with "==Problem== How many ways are there to paint each of the integers <math>2, 3, \dots, 9</math> either red, green, or blue so that each number has a different color from each o...")
 
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==Solution==
 
==Solution==
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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>.
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<math>9 \cdot 3 \cdot 2 \cdot 2 \cdot 2 \cdot 2 = \boxed{\textbf{(E) }432}</math>
  
 
==See Also==
 
==See Also==

Revision as of 18:36, 9 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

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 $9$.

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

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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