We construct a binomial distribution.
The probability of "success" (a substandard rod) is .05. We have 300 trials.
B(.05, 300)
P(X < 20) = .881
smith is a weld inspector at a ship yard. he knows from keeping track of good and substandard welds that for the afternoon shift 5% of all welds done will be substandard. If smith checks 300 of the 7500 welds completed that shift, what is the probability that he will find less than 20 substandard welds?
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