The mean deviation of the numbers $3, 4, 5, 6, 7$ is

The following data represents the frequency distribution of $20$ observations.

$x_i$	$3$	$4$	$5$	$8$	$10$	$11$
$f_i$	$\alpha+2$	$(\alpha-1)^2$	$4$	$\alpha-1$	$2$	$\alpha$

Then its mean deviation about the mean is

The mean and variance of $10$ observations are $9$ and $34.2$, respectively. If $8$ of these observations are $2, 3, 5, 10, 11, 13, 15, 21$, then the mean deviation about the median of all the $10$ observations is

The mean deviation about the mean for the following data is:

$\text{Marks obtained}$	$0-10$	$10-20$	$20-30$	$30-40$	$40-50$
$\text{Number of Boys}$	$6$	$8$	$10$	$4$	$2$

In a series of $3n$ observations,if $n$ observations are equal to $a$ and the remaining $2n$ observations are equal to $-2a$,then the mean deviation of the observations about their mean is:

If the mean deviation from the mean of the observations $\{1+K \alpha\}$,where $K=0, 1, 2, \ldots, 100$ and $\alpha > 0$ is $255$,then $\alpha =$