Find the mean, variance and standard deviation for the following data by using step deviation method:

Classes	$10-20$	$20-30$	$30-40$	$40-50$	$50-60$	$60-70$	$70-80$	$80-90$	$90-100$
Frequency	$3$	$4$	$7$	$7$	$15$	$9$	$6$	$6$	$3$

The weights of coffee in $70$ jars are shown in the following table:

Determine variance and standard deviation of the above distribution.

Two plants $A$ and $B$ of a factory show the following results about the number of workers and the wages paid to them:

In which plant, $A$ or $B$ is there greater variability in individual wages?