he numbers of defective items in successive groups of fifteen items were counted as they came off a production line. The results can be summarized as follows: No. of Defectives 0 1 2 3 4 >4 Frequency 57 57 18 5 3 0 a) Calculate the mean number of defectives in a group of fifteen items. b) Calculate the variance and standard deviation of the number of defectives in a group. Take the given data as a sample. c) Find the median, lower quartile, upper quartile, ninth decile, and 95th percentile.​