Question

The average frequency is $18$. Find the missing frequency $f$.

Class Interval	Frequency
$11-13$	$7$
$13-15$	$6$
$15-17$	$9$
$17-19$	$13$
$19-21$	$f$
$21-23$	$5$
$23-25$	$4$

Answer 1

$\begin{array}{c}\frac{7+6+9+13+f+5+4}{7}=18\\ 44+f=18\times 7\\ f=126-44=82\end{array}$