Difference between revisions of "2011 AIME II Problems/Problem 12"

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Problem:
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== Problem 12 ==
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Nine delegates, three each from three different countries, randomly select chairs at a round table that seats nine people. Let the probability that each delegate sits next to at least one delegate from another country be <math>\frac{m}{n}</math>, where <math>m</math> and <math>n</math> are relatively prime positive integers. Find <math>m + n</math>.
  
Nine delegates, three each from three different countries, randomly select chairs at a round table that seats nine people. Let the probability that each delegate sits next to at least one delegate from another country be m/n where m and n are relatively prime positive integers. Find m+n.
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== Solution ==
  
Solution:
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Use complementary probability and PIE.
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If we consider the delegates from each country to be indistinguishable and number the chairs, we have <cmath>\frac{9!}{(3!)^3}</cmath> total ways to seat the candidates. This comes to: <cmath>\frac{9\cdot8\cdot7\cdot6\cdot5\cdot4}{6\cdot6} = 6\cdot8\cdot7\cdot5 = 30\cdot56.</cmath>
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Of these there are <cmath> 3 \times 9 \times \frac{6!}{(3!)^2} </cmath> ways to have the candidates of at least some one country sit together. This comes to <cmath>\frac{27\cdot6\cdot5\cdot4}6 = 27\cdot 20.</cmath>
  
Use complementary probability and PIE.
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Among these there are <cmath> 3 \times 9 \times 4 </cmath> ways for candidates from two countries to each sit together. This comes to <cmath> 27\cdot 4. </cmath>
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Finally, there are <cmath> 9 \times 2 = 18.</cmath> ways for the candidates from all the countries to sit in three blocks (9 clockwise arrangements, and 9 counter-clockwise arrangements).
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So, by PIE, the total count of unwanted arrangements is <cmath>27\cdot 20 - 27\cdot 4 + 18 = 16\cdot27 + 18 = 18\cdot25. </cmath>
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So the fraction <cmath> \frac mn = \frac{30\cdot 56 - 18\cdot 25}{30\cdot 56} = \frac{56 - 15}{56} = \frac{41}{56}.</cmath> Thus <math>m + n = 56 + 41 = \fbox{97.}</math>

Revision as of 01:00, 2 April 2011

Problem 12

Nine delegates, three each from three different countries, randomly select chairs at a round table that seats nine people. Let the probability that each delegate sits next to at least one delegate from another country be $\frac{m}{n}$, where $m$ and $n$ are relatively prime positive integers. Find $m + n$.

Solution

Use complementary probability and PIE.

If we consider the delegates from each country to be indistinguishable and number the chairs, we have \[\frac{9!}{(3!)^3}\] total ways to seat the candidates. This comes to: \[\frac{9\cdot8\cdot7\cdot6\cdot5\cdot4}{6\cdot6} = 6\cdot8\cdot7\cdot5 = 30\cdot56.\]

Of these there are \[3 \times 9 \times \frac{6!}{(3!)^2}\] ways to have the candidates of at least some one country sit together. This comes to \[\frac{27\cdot6\cdot5\cdot4}6 = 27\cdot 20.\]

Among these there are \[3 \times 9 \times 4\] ways for candidates from two countries to each sit together. This comes to \[27\cdot 4.\]

Finally, there are \[9 \times 2 = 18.\] ways for the candidates from all the countries to sit in three blocks (9 clockwise arrangements, and 9 counter-clockwise arrangements).

So, by PIE, the total count of unwanted arrangements is \[27\cdot 20 - 27\cdot 4 + 18 = 16\cdot27 + 18 = 18\cdot25.\]

So the fraction \[\frac mn = \frac{30\cdot 56 - 18\cdot 25}{30\cdot 56} = \frac{56 - 15}{56} = \frac{41}{56}.\] Thus $m + n = 56 + 41 = \fbox{97.}$

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