3) A class has 28 students. There are 5 good students, 13 fairly good students and 10 medium students. Randomly take 4 students to attend the meeting. Find the probability to have at least 2 good students that was chosen.
The denominator of the desired probability is 28 choose 4 or 28C4.
It is easier to get the probability of the complement event and then
subtract from 1.
The complement event is that either
A. all 4 students were chosen from the 13+10 or 23 non-good students
or
B. 1 good student was chosen from the 5 and the other 3 were chosen from
the 23 non-good students.
P(A or B) = P(A)+P(B) = +
= + = 0.4324997548
That's the probability of the complement event.
The desired probability is therefore 1 - 0.4324997548 = 0.5675002452
Edwin