The math help thread

I know this is probably a really simple question, but no matter what I do I get the wrong answer! =[

So if anybody can help, that'd be great.

A laboratory measurement system used to measure blood potassium levels has a standard deviation of *sigma* = 0.2 mEq/dl in repeated testing. Repeated readings are normally distributed with mean equal to the true potassium level in the blood sample. How many times should the blood samples be tested for one person to get a margin of error no larger than *plus/minus* 0.15 mEq/dl with 95% confidence?

I have to look in my stats book but I think for confidence interval it's

sample mean/average "+ or -" sigma/SQRT(n) * Z(alpha)

Sample mean/average is "u" or "X bar"

Sigma in this problem is 0.2

SQRT(n) is square root of sample

Z(alpha) relates to 95%, in this case it's about 1.96 if i'm not mistaken

So just solve for 'n' i.e.

sigma/SQRT(n) * 1.96 = 0.15

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I might have some things wrong because I don't wanna dig up my notes, but it's just a formula somewhere.