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Find the mean deviation about the mean for the following data.

Marks obtained 10-20    20-30     30-40     40-50     50-60    60-70    70-80   
Number of students 2 3 8 14 8 3 2

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Marks obtained Number of students fi Midpoints Xi fi xi  |xi - x| fi|xi - x|
10-20 2 12 30 30 60
20-30 3 25 75 20 60
30-40 8 35 280 10 80
40-50 14 45 330 0 0
50-60 8 55 440 10 80
60-70 3 65 195 20 60
70-80 2 75 150 30 60
N = 40 = ∑f 1800 ∑f|x1 - x|2 = 400


Bar x = (∑fixi)/N = 1800/40 = 45

MD (x) = 10.

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