Answer:
probability that all the 15 students selected are girls
Step-by-step explanation:
The selection is from a sample without replacement, so we use the hypergeometric distribution to solve this question.
Hypergeometric distribution:
The probability of x sucesses is given by the following formula:
![P(X = x) = h(x,N,n,k) = \frac{C_{k,x}*C_{N-k,n-x}}{C_{N,n}}](https://tex.z-dn.net/?f=P%28X%20%3D%20x%29%20%3D%20h%28x%2CN%2Cn%2Ck%29%20%3D%20%5Cfrac%7BC_%7Bk%2Cx%7D%2AC_%7BN-k%2Cn-x%7D%7D%7BC_%7BN%2Cn%7D%7D)
In which:
x is the number of sucesses.
N is the size of the population.
n is the size of the sample.
k is the total number of desired outcomes.
Combinations formula:
is the number of different combinations of x objects from a set of n elements, given by the following formula.
![C_{n,x} = \frac{n!}{x!(n-x)!}](https://tex.z-dn.net/?f=C_%7Bn%2Cx%7D%20%3D%20%5Cfrac%7Bn%21%7D%7Bx%21%28n-x%29%21%7D)
All girls from the first group:
20 students, so ![N = 20](https://tex.z-dn.net/?f=N%20%3D%2020)
10 girls, so ![k = 10](https://tex.z-dn.net/?f=k%20%3D%2010)
5 students will be selected, so ![n = 5](https://tex.z-dn.net/?f=n%20%3D%205)
We want all of them to be girls, so we find P(X = 5).
![P_1 = P(X = 5) = h(5,20,5,10) = \frac{C_{10,5}*C_{10,5}}{C_{20,5}} = 0.0163](https://tex.z-dn.net/?f=P_1%20%3D%20P%28X%20%3D%205%29%20%3D%20h%285%2C20%2C5%2C10%29%20%3D%20%5Cfrac%7BC_%7B10%2C5%7D%2AC_%7B10%2C5%7D%7D%7BC_%7B20%2C5%7D%7D%20%3D%200.0163)
All girls from the second group:
20 students, so ![N = 20](https://tex.z-dn.net/?f=N%20%3D%2020)
5 girls, so ![k = 5](https://tex.z-dn.net/?f=k%20%3D%205)
5 students will be selected, so ![n = 5](https://tex.z-dn.net/?f=n%20%3D%205)
We want all of them to be girls, so we find P(X = 5).
![P_2 = P(X = 5) = h(5,20,5,5) = \frac{C_{5,5}*C_{15,5}}{C_{20,5}} = 0.00006](https://tex.z-dn.net/?f=P_2%20%3D%20P%28X%20%3D%205%29%20%3D%20h%285%2C20%2C5%2C5%29%20%3D%20%5Cfrac%7BC_%7B5%2C5%7D%2AC_%7B15%2C5%7D%7D%7BC_%7B20%2C5%7D%7D%20%3D%200.00006)
All girls from the third group:
20 students, so ![N = 20](https://tex.z-dn.net/?f=N%20%3D%2020)
8 girls, so ![k = 8](https://tex.z-dn.net/?f=k%20%3D%208)
5 students will be selected, so ![n = 5](https://tex.z-dn.net/?f=n%20%3D%205)
We want all of them to be girls, so we find P(X = 5).
![P_3 = P(X = 5) = h(5,20,5,8) = \frac{C_{8,5}*C_{12,5}}{C_{20,5}} = 0.0036](https://tex.z-dn.net/?f=P_3%20%3D%20P%28X%20%3D%205%29%20%3D%20h%285%2C20%2C5%2C8%29%20%3D%20%5Cfrac%7BC_%7B8%2C5%7D%2AC_%7B12%2C5%7D%7D%7BC_%7B20%2C5%7D%7D%20%3D%200.0036)
All 15 students are girls:
Groups are independent, so we multiply the probabilities:
![P = P_1*P_2*P_3 = 0.0163*0.00006*0.0036 = 3.52 \times 10^{-9}](https://tex.z-dn.net/?f=P%20%3D%20P_1%2AP_2%2AP_3%20%3D%200.0163%2A0.00006%2A0.0036%20%3D%203.52%20%5Ctimes%2010%5E%7B-9%7D)
probability that all the 15 students selected are girls