Random sample of 120 customersís spent an average of 9.2 hours on their professional job with a sample standard deviation of 5.1 hours. Calculate the specification hours with confidence of 95%.
The Central Limit Theorem (which the general rule of thumb says is ok to invoke for ) tells us that is approximately normally distributed with mean 0 and std dev 1.
So then the confidence interval is given by where is defined by (Z is from the standard normal distribution). I'm assuming that you have already seen where these formulas came from.
So from a standard normal distribution table . So just plug away.