7.26 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 (See Exercise 7.1), find the margin of error of a 95% confidence interval for the mean speed of all automobiles traveling over this stretch of highway.