2011 AIME II Problems/Problem 12
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 , where
and
are relatively prime positive integers. Find
.
Solution
Use complementary probability and PIE.
If we consider the delegates from each country to be indistinguishable and number the chairs, we have total ways to seat the candidates. This comes to:
Of these there are ways to have the candidates of at least some one country sit together. This comes to
Among these there are ways for candidates from two countries to each sit together. This comes to
Finally, there are 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
So the fraction Thus