If the mean of a set of observations $x_1, x_2, \dots, x_{10}$ is $20$,then find the mean of $x_1 + 4, x_2 + 8, x_3 + 12, \dots, x_{10} + 40$.

$x_1, x_2, \dots, x_{34}$ are numbers such that $x_i = 150$ for all $i \in \{1, 2, \dots, 10\}$ and $x_{i+1} - x_i = -2$ for all $i \in \{10, 11, \dots, 33\}$. Find the median of $x_1, x_2, \dots, x_{34}$.

Find the median for the following distribution:

Class	Frequency
$10-20$	$180$
$20-30$	$82$
$30-40$	$34$
$40-50$	$180$
$50-60$	$136$
$60-70$	$23$
$70-80$	$50$

What is the median of an ordered series of $n$ even observations?

If the mean of the first $n$ natural numbers is equal to $\frac{n + 7}{3}$,then what is the value of $n$?

In a class of $100$ students,there are $70$ boys whose average marks in a subject are $75$. If the average marks of the complete class are $72$,then what are the average marks of the girls?