Solution:
Given: Scores on the Math portion of the SAT are believed to be
Normally distributed and range from 200 to 800.
c = confidence level = 90%
Margin of Error = E = 23
We have to find sample size n.

Population standard deviation
is not given, but we can estimate it using Range.





and
Zc is z critical value for c = 90% confidence level.
Find Area = ( 1 + c ) / 2 = ( 1 + 0.90) / 2 = 1.90 / 2 = 0.9500
Look in z table for Area = 0.9500 or its closest area and find corresponding z value.

Area 0.9500 is in between 0.9495 and 0.9505 and both the area are at same distance from 0.9500
Thus we look for both area and find both z values
Thus Area 0.9495 corresponds to 1.64 and 0.9505 corresponds to 1.65
Thus average of both z values is : ( 1.64+1.65) / 2 = 1.645
Thus Zc = 1.645
Thus





( Sample size is always rounded up)
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