Question
If the arithmetic mean of the data given below is 28, find (a) the missing frequency, and (b) the median of the series:
Profit per retail shop (in ₹)
0-10
10-20
20-30
30-40
40-50
50-60
Number of retail shops
12
18
27
-
17
6

Answer

Let us take the missing frequency as x.
Profit per retail shop (in ₹) (X)
Number of retail shops (f)
Mid values (m)
fm
0-10
10-20
20-30
30-40
40-50
50-60
12
18
27
x
17
6
5
15
25
35
45
55
60
270
675
35x
765
330
 
$\sum\text{f} = 80 +\text{x}$
 
$\sum\text{Fm} = 2100 + 35\text{x}$
$\text{Mean}=\frac{\sum\text{fm}}{\sum\text{f}}$
$\text{Mean}=28$
Substituting the values in the formula we get,$28=\frac{2100\ +\ 35\text{x}}{80\ +\ \text{x}}$
$28\text{x}(80+\text{x})=2100+35\text{x}$
$2240+28\text{x}=2100+35\text{x}$
$7\text{x}=140\text{x}$
$\text{x}=20$
Thus, the missing value frequency is 20.
Class Interval
Frequency (f)
Cumulative frequency (CF)
0-10
12
12
10-20
18
30
20-30
27
57
30-40
x
77
40-50
17
94
50-60
6
100
Total
$\sum\text{f}=100$
 
Formula of median is as follow: Median$=\text{L}+\frac{\frac{\text{N}}{2}-\text{Cf}}{\text{f}}\times\text{h}$
By substituting the value in the formula we get,$\text{Median}=20+\frac{(50-30)}{27}\times(30-20)$
$\text{Median}=20+\frac{(20)}{27}\times(10)$
$\text{Median}=20+\frac{200}{27}$
$\text{Median}=20+7.41$
$\text{Median}=27.41$
Thus the median value of the series is 27.41

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