First, find the critical value of Z for a 90% confidence interval:
Norm.inv ( 0.95, 0, 1 ) = 1.64
Standard deviation is sqrt(variance) = sqrt(5.76) = 2.40
Remember that the margin of error will be given by Z_crit x standard error:
Margin of error = 1.64 x sd/sqrt(n)
Rearrange in terms of n:
sqrt(n) = 1.64 x sd / Margin of error
n = [ 1.64 x 2.40 / 0.11 ] ^2
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