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NCERT Class XII Mathematics Chapter - - Solutions

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Question : 80
Total: 101
It is known that 10% of certain articles manufactured are defective. What is the probability that in a random sample of 12 such articles, 9 are defective?
Solution:  
Let p be the probability of success of defective article = 10% =
1
10

⇒q=1−
1
10
=
9
10

Let X has a binomial distribution with n=12,p=
1
10
,q=
9
10

∴P(X=r)=nCr(q)n−rpr
∴ P (9 defective) = P(X = 9)
=12C9q3p9=12C9(
9
10
)3(
1
10
)9
=
22×93
1011
.
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