There is concern about the speed of automobiles traveling over a particular stretch of highway. For a random sample of 28 automobiles, radar indicated the following speeds, in miles per hour:
59 63 68 57 56 71 59
69 53 58 60 66 51 59
54 64 58 57 66 61 65
70 63 65 57 56 61 59
Assuming a normal population distribution, find the margin of error of a 95% confidence interval for the mean speed of all automobiles traveling over this stretch of highway.