statistics
posted by terri .
The ages of five randomly chosen cars in the parking lot are determined to be 7,9,3,4 and 6 years old. If we consider this sample in groups of 3, what is the probability of the true mean falling between 5.5 and 6.5?

I'm not sure what you mean by considering "in groups of 3."
Find the mean first = sum of scores/number of scores
Subtract each of the scores from the mean and square each difference. Find the sum of these squares. Divide that by the number of scores to get variance.
Standard deviation = square root of variance
Z = (scoremean)/SEm
SEm = SD/√n (Are you saying n = 3? If not, n = 5.)
Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability related to your Z scores.
I'll let you do the calculations.