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
1Nn∑i=1fi|xi−¯x|
Clearly mean of the given distribution will be
x=n∑i=1fi xin∑i=1 fi
To find the mean deviation about ¯x we need sum of absolute deviation about ¯x for each xi
Which would be ∑ni=1fi(xi−x|
Total number of observations = n∑i=1 fi = N
∴ Mean deviation about mean = =1N∑ni=1fi(xi−x|
∴ option b