If the mean of the frequency distribution is $28$,then its variance is $........$.

Class	$0-10$	$10-20$	$20-30$	$30-40$	$40-50$
Frequency	$2$	$3$	$x$	$5$	$4$

Two dice $A$ and $B$ are rolled. Let the numbers obtained on $A$ and $B$ be $\alpha$ and $\beta$ respectively. If the variance of $\alpha - \beta$ is $\frac{p}{q}$,where $p$ and $q$ are coprime,then the sum of the positive divisors of $p$ is equal to

The $S.D.$ of a variate $x$ is $\sigma$. The $S.D.$ of the variate $\frac{ax + b}{c}$,where $a, b, c$ are constants,is

For a frequency distribution,the standard deviation is computed by:

If $\sum_{i=1}^{5}(x_i-10)=5$ and $\sum_{i=1}^{5}(x_i-10)^2=5$,then the standard deviation of the observations $2x_1 + 7, 2x_2 + 7, 2x_3 + 7, 2x_4 + 7,$ and $2x_5 + 7$ is equal to-

The variance of $20$ observations is $5$. If each of the observations is multiplied by $2$,then the variance of the resulting observations is: