For the given distinct values x1,x2,x3,.....xn occurring with frequencies f1,f2,f3,...fn respectively. The mean deviation about mean where ¯¯¯x is the mean and N is total number of observations would be
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Clearly mean of the given distribution will be ¯¯¯x = ∑ni=1 fi xi∑ni=1 fi
To find the mean deviation about ¯¯¯x we need sum of absolute deviation about ¯¯¯x for each xi
Which would be ∑ni=1 fi | xi - ¯¯¯x|
Total number of observations = ∑ni=1 fi = N
∴ Mean deviation about mean = 1N∑ni=1 fi | xi - ¯x|
∴ option b