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The track team gives awards for first, second, and third place runners. There are 10 students from school A and 12 students from school B competing.

Which expression represents the probability that all three awards will go to a student from school B?

A. [tex]\(\frac{12^{P_3}}{22^{P_3}}\)[/tex]
B. [tex]\(\frac{12 C _3}{{ }_{22} C _3}\)[/tex]
C. [tex]\(\frac{22 P_3}{22 P_{12}}\)[/tex]
D. [tex]\(\frac{{ }_{22} C_3}{{ }_{22} C_{12}}\)[/tex]


Sagot :

Let's solve this step-by-step to determine the expression representing the probability that all three awards will go to students from school B.

1. Total Number of Students:
There are 10 students from school A and 12 students from school B, making a total of 22 students.

2. Objective:
We need to find the probability that all three awards (first, second, and third place) will go to students from school B.

3. Concepts Used:
- Permutations (P): Denoted as [tex]\( nP_r \)[/tex], which represents the number of ways to arrange [tex]\( r \)[/tex] objects out of [tex]\( n \)[/tex] objects.
- Combinations (C): Denoted as [tex]\( nC_r \)[/tex], which represents the number of ways to choose [tex]\( r \)[/tex] objects out of [tex]\( n \)[/tex] objects without regard to the order.

4. Calculation of Desired Probability:
- Permutations:
- The number of ways to choose and arrange 3 students out of the 12 students from school B is [tex]\(\text{perm}(12, 3)\)[/tex].
- The number of ways to choose and arrange 3 students out of the total 22 students is [tex]\(\text{perm}(22, 3)\)[/tex].
- The probability is then the ratio of these two permutations:
[tex]\[ \frac{\text{perm}(12, 3)}{\text{perm}(22, 3)} \][/tex]

5. Simplify the Expression:
[tex]\[ \frac{12 \times 11 \times 10}{22 \times 21 \times 20} \][/tex]
However, we do not need to simplify it further for this step.

6. Finding the Correct Expression:
Looking at the provided options:
- [tex]\(\frac{{12}^{P_3}}{22^{P_3}}\)[/tex] (Not a standard notation)
- [tex]\(\frac{{12 C _3}}{22 C_ 3}\)[/tex] (Using combinations: incorrect)
- [tex]\(\frac{22 P_3}{22 P_{12}}\)[/tex] (Incorrect as it doesn't make sense contextually)
- [tex]\(\frac{{22 C_ 3}}{22 C_{12}}\)[/tex] (Incorrect use of combinations for different purposes)

The correct expression should represent the ratio of the number of permutations of choosing 3 students from 12 to the number of permutations of choosing 3 students from 22.

Thus, the correct expression is:
[tex]\[ \frac{\text{perm}(12, 3)}{\text{perm}(22, 3)} \][/tex]

Given the answer derived above (the numerical result), this probability is:
[tex]\[ 0.14285714285714285 \][/tex]
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