Construct a 90% confidence interval for the population mean, µ. assume the population has a normal distribution. a sample of 15 randomly selected math majors has a grade point average of 2.86 with a standard deviation of 0.78. round to the nearest hundredth

Confidence interval of mean = point average (p) +/- Z*sd/Sqrt (sample size)

p = 2.86, Z at 90%confidence = 1.645, Sample size = 15, sd = 0.78

Substituting;

Confidence interval = 2.86 +/- 1.645*0.78/Sqrt (15) = 2.86 +/- 0.33 = {(2.86-0.33),(2.86+0.33)] = {2.53,3.19}

What am i doing here. add?