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:
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.
All girls from the first group:
20 students, so 
10 girls, so 
5 students will be selected, so 
We want all of them to be girls, so we find P(X = 5).
All girls from the second group:
20 students, so
5 girls, so 
5 students will be selected, so
We want all of them to be girls, so we find P(X = 5).
All girls from the third group:
20 students, so
8 girls, so 
5 students will be selected, so
We want all of them to be girls, so we find P(X = 5).
All 15 students are girls:
Groups are independent, so we multiply the probabilities:
probability that all the 15 students selected are girls
