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Math Help - STATS HELP!

  1. #1
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    STATS HELP!

    10) Smith is a weld inspector at a shipyard. 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?
    A) 0.5934 B) 0.0934 C) 0.4066 D) 0.9073
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  2. #2
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    Quote Originally Posted by nrichelle7 View Post
    10) Smith is a weld inspector at a shipyard. 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?
    A) 0.5934 B) 0.0934 C) 0.4066 D) 0.9073
    Since you're sampling from a large population, I'd have thought you could just use the Poisson distribution with mean 15 (5% of 300) to calculate Pr(X < 20) .....

    (If you're not using technology that can perform this calculation, I'd suggest using the normal approximation to the Poisson distribution.)

    Unfortunately I get 0.8752 - which doesn't match any of the given options.
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