# STATS HELP!

• Oct 4th 2008, 01:07 PM
nrichelle7
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
• Oct 5th 2008, 05:02 AM
mr fantastic
Quote:

Originally Posted by nrichelle7
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.