# There are 5% defective items in a large bulk of items. What is probability that a sample of 10 items will include not more than one defective item? - Mathematics and Statistics

Sum

There are 5% defective items in a large bulk of items. What is the probability that a sample of 10 items will include not more than one defective item?

#### Solution

Let X = number of defective items

p = probability of defective item

∴ p = 5% = 5/100 = 1/20

and q = 1 - p = 1 - 1/20 = 19/20

∴ X ~ B (10, 1/20)

The p.m.f. of X is given by

P(X = x) = "^nC_x  p^x  q^(n - x)

i.e. p(x) = "^10C_x  (1/20)^x (19/20)^(10 - x), x = 0, 1, 2, ...,10

P(sample of 10 items will include not more than one defective item) = P[X ≤ 1]

= P(x = 0) + P(x = 1)

= ""^10C_0 (1/20)^0(19/20)^(10 - 0) + "^10C_1 (1/20)^1 (19/20)^(10 - 1)

= 1*1*(19/20)^10 + 10 xx (1/20) xx (19/20)^9

= (19/20)^9 [19/20 + 10/20]

= (19/20)^9 (29/20) = 29(19^9/20^10)

Hence, the probability that a sample of 10 items will include not more than one defective item = 29 (19^9/20^10).

Concept: Binomial Distribution
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