Answer:
Margin of Error (half of confidence interval) = 10
The margin of error is defined as the "radius" (or half the width) of a confidence interval for a particular statistic.
Level of Confidence = 95
Ď: population standard deviation = 40
('z critical value') from Look-up Table for 95% = 1.96
The Look-up in the Table for the Standard Normal Distribution utilizes the Table's cummulative 'area' feature. The Table shows positve and negative values of ('z critical') but since the Standard Normal Distribution is symmetric, only the magnitude of ('z critical') is important.
