Question 92338
A simple random sample is to be drawn from a population of 1200, in order to have 90%confidence that the sampling error in estimating "n"is no more the 0.03, what sample size will be necessary? 
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Since E=z'[sigma/sqrt(n)] 

n=[z'/E]^2

Your Problem:

n=[1.645*sigma/0.03]^2

n = [1.645/0.03]^2 *sigma^2

n = 3006*sigma^2

Comment: You would have to know sigma to arrive at "n".
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Cheers,
Stan H.