# Prob/Stats

Historically, 5 percent of a mail-order firm's repeat charge-account customers have an incorrect current address in the firm's computer database. a)what is the probability that none of the next 12 repeat customers who call will have an incorrect address? b) One custome...
Tuesday, October 20, 2009 at 7:17pm by

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1. probability of an incorrect address = 5/100

to have atleast 1 incorrect address out of the 12 customers (choice of 1 out of 12 customers)

12C1 x (5/100) =

(12!/ 1! 11!) (5/100)

= 12 x (5/100) = 60/100

So for none to be incorrect it will be (1 - Prob of atleast one to be incorrect)

1- (60/100) = 40/100 = 2/5

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2. I beg to differ from Visper.

Probability of having a correct address is (95/100). So, probability of correct 12 in a row (or none incorrect) is (95/100)^12 = .54

So, probability of having AT LEAST one incorrect = 1-.54 = .44

Probability of having exactly one incorrect is 12C1 * (5/100) * (95/100)^11
= 12 * .05 * .5688 = .3413

I hope this helps.

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