Question

A class contains 10 sophomores, 15 juniors, and 20 seniors. A committee of 5 students is to be chosen. Each student is equally likely to be selected to serve on the committee. Let A be the event exactly 3 juniors are selected to serve on the committee. Let B be the event exactly 2 seniors are selected to serve on the committee.

Required:
a. Find P(A)
b. Find P(A| B)

Answer 1

P(A) = 0.1620 , P(A|B) = 0.0707

Explanation:

Total people = 10 + 15 + 20 = 45 , Sophomores = 10, Juniors = 15 , Seniors = 20

P(A) , exactly 3 juniors selected = (15 c 3) (30 c 2) / (45 c 5) = (455 x 435) / 1221759 = 0.1620

P (B) , exactly 2 seniors selected = (20 c 2) (25 c 3) / (45 c 5) = (190 x 2300)/ 1221759 = 0.3576

P(A|B), exactly 3 juniors & exactly 2 seniors selected = P (A∩B) / P (B)

(15 c 3) (20 c 2) / (45 c 5) = (455 x 190) / 1221759 = 0.0707