University of South Carolina High School Math Contest/1993 Exam/Problem 14
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.
![$\mathrm{(A) \ }9\cdot 7! \qquad \mathrm{(B) \ } 8! \qquad \mathrm{(C) \ }5!4! \qquad \mathrm{(D) \ }8!4! \qquad \mathrm{(E) \ }8!+6!+4!$](http://latex.artofproblemsolving.com/e/c/7/ec73868eb5d35d371c3578d6d7e3eae1b1b2d23d.png)
Solution
For the nine slots, we have to choose two spots for each of our pairs . There are
ways to do this, but we must multiply this by
because there are three different pairs that we can choose in different "orders" for the
. We also need to multiply by
again for the remaining three slots for
. Thus the total number of ways is
, which after simplifying is choice
.