90% CONFIDENCE FOR THE MEAN TIME TO COMPLETE A TEST. POPULATION STANDARD DEVIATION OF 3 HOURS. SAMPLE SIZE IF THE ERROR SHOULD BE LESS THAN A HALF HOUR?
90% = mean ± Z (SEm)
Look in the back of your statistics textbook for a table called something like “area under normal distribution” to find the proportion/probability (± .05) and its Z score.
90% = mean ± 1.645 SEm
.5 = 1.645 SEm
SEm = SD/√n
Solve for n.