The speeds of 55 cars were measured by a radar device on a city street:
a. Classify these data into a grouped frequency distribution by using class boundaries 12–18, 18–24, . . . , 48–54.
b. Find the class width.
c. For the class 24–30, find the class midpoint, the lower class boundary, and the upper class boundary. d. Construct a frequency histogram of these data.