Difference between revisions of "University of South Carolina High School Math Contest/1993 Exam/Problem 11"
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+ | Using the formula for [[derangements]], <math><cmath>!n = n! \sum_{k=0}^{n} \frac{(-1)^k}{k!}.</cmath></math>, we can substitute in our values to find that the number of derangements is <math>24\cdot\frac{1}{1}-\frac{1}{1}+\frac{1}{2}-\frac{1}{6}+\frac{1}{24}.</math> Simplifying, we get <math>\frac 38 \Longrightarrow \mathrm{(B)}</math>. | ||
* [[University of South Carolina High School Math Contest/1993 Exam/Problem 10|Previous Problem]] | * [[University of South Carolina High School Math Contest/1993 Exam/Problem 10|Previous Problem]] | ||
* [[University of South Carolina High School Math Contest/1993 Exam/Problem 12|Next Problem]] | * [[University of South Carolina High School Math Contest/1993 Exam/Problem 12|Next Problem]] |
Revision as of 10:03, 13 February 2018
Problem
Suppose that 4 cards labeled 1 to 4 are placed randomly into 4 boxes also labeled 1 to 4, one card per box. What is the probability that no card gets placed into a box having the same label as the card?
Solution
This is the number of derangements of 4 objects. We can know the formula for derangements or count in one of two ways:
Counting directly: The card labeled 1 has 3 places to go. Without loss of generality we may say that it goes in the place marked 2. Now, if card 2 goes into box 1, we have only one possibility because cards 3 and 4 must be interchanged. Otherwise, there are 2 possibilities for card 2, and each of these leads to one more possible arrangement (if card 2 goes in box 3, card 3 must go in box 4 and card 4 in box 1, while if card 2 goes in box 4, card 4 must go in box 3, and card 3 must go in box 1). This gives us a total of good arrangements. (Equivalently, one could say that the only permutations of 4 objects with no fixed points are those with cycle notation and , of which there are 6 and 3, respectively.) Thus the probability is .
Counting the complement:
Solution 2
Using the formula for derangements, , we can substitute in our values to find that the number of derangements is Simplifying, we get .