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The Mean of the Following Distribution is 18. Find the Frequency F Of Class 19 – 21. - Mathematics

The mean of the following distribution is 18. Find the frequency f of class 19 – 21.

Class 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

Class Frequency fi  Mid value xi `f_ix_i`
11-13 3 12 36
13-15 6 14 84
15-17 9 16 144
17-19 13 18 234
19-21 f 20 20f
21-23 5 22 110
23-25 4 24 96

Mean = `(sumf_ix_i)/(sum f_i) = (704 + 20f)/(40 + f) = 18 `

`=> 704 + 20f = 720 + 18f`

`=> 2f = 16` 

`=> f = 8`

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