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Question

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

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Solution

It is given that mean of the data is 18.
Let the assumed mean A = 18 and h = 2.
Marks Mid-Value(xi) Frequency ui=xi-182 fiui
11–13 12 3 −3 −9
13–15 14 6 −2 −12
15–17 16 9 −1 −9
17–19 18 13 0 0
19–21 20 f 1 f
21–23 22 5 2 10
23–25 24 4 3 12
N = 40 + f fiui=-8+f

We know that mean,
Now, we have N=fi=40+f, h=2, A=18, fiui=-8+f
18=18+2140+f×-8+f0=-16+2f40+f-16+2f=0f=8


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