Posted by **Anshu** on Tuesday, August 27, 2013 at 4:26am.

Approximately 15% of the students in ST130

obtain A grade. If 3 students are selected at

random, find the probability that

a)all of them obtain A grade in ST130.

b)Exactly 2 obtain A grade in ST130.

c)at least one obtain A grade in ST130

- Math -
**Reiny**, Tuesday, August 27, 2013 at 8:16am
prob of A = .15

prob of notA = .85

a) prob all 3 to get A = (.15)^3 = .003375

b) exactly 2 A's = C(3,2) .15^2 (.85)

= .057375

c) at least one

= 1 - prob no A's

= 1 - .85^3 = .385875

or the long way:

prob oneA + prob 2 A's + prob 3 A'a

= C(3,1) (.15)(.85)^2 + C(3,2) (15)^2 (.85) + C(3,3) (.15)^3

= .325125 + .057375 + .003375

= .385875

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