The average age of a teacher and three students is $20$ years. If all the students are of the same age and the difference between the teacher's age and the age of one of the students is $20$ years,find the teacher's age in years.

If for some $x \in R$,the frequency distribution of the marks obtained by $20$ students in a test is:
Marks: $2, 3, 5, 7$
Frequency: $(x+1)^2, 2x-5, x^2-3x, x$
Then the mean of the marks is:

If in an examination different weights are assigned to different subjects: Physics $(2)$,Chemistry $(1)$,English $(1)$,and Mathematics $(2)$. If a student scored $60$ in Physics,$70$ in Chemistry,$70$ in English,and $80$ in Mathematics,then his weighted $A.M.$ is:

The reciprocal of the mean of the reciprocals of $n$ observations is their

Which of the following averages is most affected by extreme observations?

The median of the following frequency distribution is:

$x_i$	$3$	$6$	$10$	$12$	$7$	$15$
$f_i$	$3$	$4$	$2$	$8$	$13$	$10$