If the mean of the following frequency distribution is 18, find the missing frequency.

Class interval:	11-13	13-15	15-17	17-19	19-21	21-23	23-25
Frequency:	3	6	9	13	$f$	5	4.

Given:

The mean of the given frequency distribution is 18.

To do:

We have to find the missing frequency.

Solution:

Let the assumed mean be $A=18$.

Class size $h=2$

Mean $=A+h \times \frac{\sum{f_iu_i}}{\sum{f_i}}$    

Therefore,  

Mean $18=18+2\times(\frac{f-8}{40+f})$

$18-18=2\times\frac{f-8}{40+f}$

$\frac{f-8}{40+f}=0$

$f-8=0$

$f=8$

The missing frequency is 8.

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Updated on: 10-Oct-2022

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