A lot of 10 components contains 3 that are defective. Two components are drawn at random and tested. Let A be the event that the first component drawn is defective, and let B be the event that the second component drawn is defective.

a. Find P(A).

b. Find P(B|A).

c. Find P(A ? B).

d. Find P( ? B).

e. Find P(B).

f. Are A and B independent? Explain.


(a) P(A) = 3?10



(b) Given that A occurs, there are 9 components remaining, of which 2 are defective.

Therefore P(B|A) = 2?9.



(c) P(A ? B) = P(A)P(B|A) = (3?10)(2?9) = 1?15



(d) Given that occurs, there are 9 components remaining, of which 3 are defective.

Therefore P(B|A) = 3?9. Now = (7?10)(3?9) = 7?30.



(e) P(B) = P(A ? B) + P(? B) = 1?15 + 7?30 = 3?10



(f) No. P(B) ? P(B|A) [or equivalently, P(A ? B) ? P(A)P(B)].

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