If the mean of five observations $x, x + 2, x + 4, x + 6$,and $x + 8$ is $11$,what is the mean of the last three observations?

If the mean of the series $x_1, x_2, ......., x_n$ is $\bar{x}$,then what will be the mean of the series $x_i + 2i, (i = 1, 2, ......., n)$?

If four different subjects are assigned weights in an examination as Physics $(2)$,Chemistry $(1)$,English $(1)$,and Mathematics $(2)$,and a student scores $60$ in Physics,$70$ in Chemistry,$70$ in English,and $80$ in Mathematics,find the weighted mean.

The mean weight per student in a group of seven students is $55 \ kg$. If the individual weights of $6$ students are $52, 58, 55, 53, 56$ and $54 \ kg$,then the weight of the seventh student is.....$kg$.

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

The arithmetic mean of the first $n$ natural numbers is: