Using step - deviation method, calculate the mean marks of the following distribution.
Class intervalFrequency50−55555−602060−651065−701070−75975−80680−851285−908
Class intervalMid−valuefidi=xi−Aui=dihfi×ui(xi)(h=5)50−5552.55−15−3−1555−6057.520−10−2−4060−6562.510−5−1−1065−70A=67.51000070−7572.5951975−8077.561021280−8582.5121533685−9087.5820432
Mean (¯x)=A+∑fiui∑fi×h=67.5+2416=67.5+1.5=69.0