Question 1183098
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sigma = 30.8


At 95% confidence, the critical z value is roughly z = 1.960 (use a table or calculator to find this). 


The error we want to have is E = 1.5


Now we can compute the min sample size needed
n = (z*sigma/E)^2
n = (1.960*30.8/1.5)^2
n = 1619.68685511111
n = 1620
Always round up to ensure we clear the hurdle.


Answer: 1620
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