The following is the frequency distribution for the speeds of a sample of automobiles traveling on an interstate highway.
speed
miles per hour frequency
50 - 54 2
55 - 59 4
60 - 64 5
65 - 69 10
70 - 74 9
75 - 79 5
refer to exhibit 3-4. the mean is
(a) 35
(b) 670
(c) 10
(d) 67
(e) none of the above answers is correct.