The central value of the set of observations is called

The mean of the series $a, a + nd, a + 2nd$ is

What is the value of $x$ if the mean of $8, 6, 7, 5, x,$ and $4$ is $7$?

Consider a set of observations ${x_1}, {x_2}, {x_3}, ..., {x_{101}}$ such that ${x_1} < {x_2} < {x_3} < ... < {x_{100}} < {x_{101}}$. The mean deviation of this set of observations about a point $k$ is minimum when $k$ equals:

Find the arithmetic mean of the following frequency distribution.

$x_i$	$5$	$8$	$11$	$14$	$17$
$f_i$	$4$	$5$	$6$	$10$	$20$

$A$ student obtains $75\%$,$80\%$,and $85\%$ in three subjects. If the marks of a fourth subject are added,then his average cannot be less than.....$\%$