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The following distribution shows the daily pocket allowance given to the children of a multistory building. The average pocket allowance is Rs. 18.00. Find out the missing frequency.

 Class interval:  11 - 13  13 - 15  15 - 17  17 -19  19 - 21  21 - 23  23 - 25
 Frequency:  7  6  9  13 ....  5  4

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Best answer

Given, 

Mean = 18

Let missing frequency be V

 Class interval  Mid value (xi)  fi  fixi
 11 - 13  12  7  84
 13 - 15  14  6  84
 15 - 17  16  9  144
 19 - 21  20  V 20V
 21 - 23  22  5  110
 23 - 25  24  4  56
 N = 44 + V \(\sum\)fixi = 752 + 20V

Mean = \(\frac{\sum f_ix_i}{N}\)

 \(18 = \frac{752+20V}{44+V}\)

792 + 18V = 752 + 20V 

792 – 752 = 20V – 18V 

40 = 2V 

V = 20

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