Use the grouped data formulas to find the indicated mean or standard deviation. The speeds of a random sample of 100 cars are recorded as they pass a highway checkpoint. The results are summarized in the frequency distribution below. Approximate the sample mean. Speed (mph) | Cars 30-39 6 40-49 15 50-59 51 60-69 15 70-79 13 55.9 mph 58.7 mph 54.5 mph O 61.5 mph