First we find the mean of the data set: M=(4+8+13+15+24+25+28)/7=16.7. Now we find the distance of each value from the mean value:
16.7-4=12.7, 16.7-8=8.7, 16.7-13=3.7, 16.7-15=1.7, 24-16.7=7.3, 25-16.7=8.3 and 28-16.7=11.3. Now we find the mean absolute deviation by finding the mean value of the distances: (12.7+8.7+3.7+1.7+7.3+8.3+11.3)/7=53.7/7=7.67. So the mean absolute value is A) 7.67.
The mistake was that in calculating only the mean value, not the mean absolute deviation.
