Let $M$ denote the median of the following frequency distribution. Then $20M$ is equal to:

Class	$0-4$	$4-8$	$8-12$	$12-16$	$16-20$
Frequency	$3$	$9$	$10$	$8$	$6$

The median of $10, 14, 11, 9, 8, 12, 6$ is

Compute the median from the following table:

Marks obtained	No. of students
$0-10$	$2$
$10-20$	$18$
$20-30$	$30$
$30-40$	$45$
$40-50$	$35$
$50-60$	$20$
$60-70$	$6$
$70-80$	$3$

$A$ school has four sections of chemistry in class $XII$ having $40, 35, 45$ and $42$ students. The mean marks obtained in chemistry test are $50, 60, 55$ and $45$ respectively for the four sections. The overall average of marks per student is:

Find the weighted mean of the first $n$ natural numbers,where the weights are equal to their squares.

If for some positive $x \in R$,the frequency distribution of the marks obtained by $20$ students in a certain test is as follows:

Marks	$2$	$3$	$5$	$7$
Frequency	$(x+1)^2$	$2x-5$	$x^2-3x$	$x$

Then the mean of the marks is: