74. Automobiles traveling on a road with a posted speed limit of 55 miles per hour are checked for speed by a state police radar system. Following is a frequency distribution of speeds. Speed (miles per hour) 4549 50-54 55-59 60-64 6569 70-74 75-79 Frequency 10 40 150 175 75 15 10 Total 475 a. What is the mean speed of the automobiles traveling on this road. b. Compute the variance and the standard deviation