Using shortcut method, find the standard deviation for the data:

Class	$25-35$	$35-45$	$45-55$	$55-65$	$65-75$
Frequency	$64$	$132$	$153$	$140$	$51$

 

The mean and variance of five observations are $6$ and $4$ respectively. If three of these are $5,7$ and $9$, find the other two observations.

The mean and standard deviations of a group of $100$ observations were found to be $20$ and $3$ respectively. Later on it was found that three observations $21,12$ and $18$ were incorrect. Find the mean and standard deviation if the incorrect observations were omitted.