Difference between revisions of "University of South Carolina High School Math Contest/1993 Exam/Problem 14"
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− | + | There are <math>9!</math> (the [[factorial]] of 9) total [[permutation]]s of the [[element]]s of that [[set]]. 1 is to the left of 2 in exactly half of these. 3 is also to the left of 4 in exactly half of the permutations, and 5 is to the left of 6 in exactly half of the permutations. These three events are totally independent of each other, so the number we want to calculate is <math>\frac12\cdot\frac12\cdot\frac12\cdot9! = \frac18\cdot9\cdot8\cdot7! = 9\cdot7!</math> which is answer choice <math>\mathrm{(A)}</math>. | |
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+ | Alternatively, note that we can choose <math>{9\choose 2}</math> places for the 1 and 2, then <math>{7\choose2}</math> places for the 3 and 4, then <math>5\choose 2</math> places for the 5 and 6, and the arrange the 7, 8 and 9 in <math>3!</math> ways, giving us a total of <math>{9\choose2}\cdot{7\choose2}\cdot{5\choose2}\cdot3! = \frac{(9\cdot8)\cdot(7\cdot6)\cdot(5\cdot4)\cdot3!}{2\cdot2\cdot2} = 9\cdot7! \Longrightarrow \mathrm{(A)}</math>. | ||
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Latest revision as of 16:54, 17 August 2006
Problem
How many permutations of 1, 2, 3, 4, 5, 6, 7, 8, 9 have:
- 1 appearing somewhere to the left of 2,
- 3 somewhere to the left of 4, and
- 5 somewhere to the left of 6?
For example, 8 1 5 7 2 3 9 4 6 would be such a permutation.
Solution
There are (the factorial of 9) total permutations of the elements of that set. 1 is to the left of 2 in exactly half of these. 3 is also to the left of 4 in exactly half of the permutations, and 5 is to the left of 6 in exactly half of the permutations. These three events are totally independent of each other, so the number we want to calculate is which is answer choice .
Alternatively, note that we can choose places for the 1 and 2, then places for the 3 and 4, then places for the 5 and 6, and the arrange the 7, 8 and 9 in ways, giving us a total of .