1.

40% of the buttons manufactured by a firm are found to be defective. In a random sample of six buttons, find the probability of getting.(i) exactly 4 defective buttons (ii) more than one defective button.

Answer»

Let X be the number of buttons manufactured is a Binomial.

Variate with parameters n = 6 and p = 40% = 0.4, so q = 1 – p = 0.6. 

Then the probability mass function is:

p (x) = nCx px qn-x = 0, 1, 2 ……. n.

= 6Cx (0.4)x (0.6)6-x ; x = 0, 1, 2 6.

(i) P(exactly 4 defective buttons) = p(x = 4)

= 6C4 (0.4)4 (0.6)6-4 = 15 × 0.0256 × 0.36 

= 0.1382

(ii) P(more than 1 defective button) 

=p(x > 1) = 1 -p(x = 0)

= 6C0 (0.4)0 (0.6)6-0 = 1.1. 0.466 

= 0.0466.



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