Question
Calculate the missing frequency from the following distribution, it being given that the median of the distribution is 24.
Age (in years)
0-10
10-20
20-30
30-40
40-50
Number of persons
5
25
?
18
7

Answer

Age (in years)
Number of persons (f)
Cumulative frequency (cf)
0-10
5
5
10-20
25
30
20-30
x
30 + x
30-40
18
48 + x
40-50
7
55 + x
Median = 24
Hence, median class is 20-30
$\therefore\text{l}=20,\ \text{h}=10,\ \text{f}=\text{a},\ \text{cf}=$ cf of preceding class $=30,\ \frac{\text{N}}{2}=\frac{55+\text{a}}{2}$
Now, median $=\text{l}+\begin{Bmatrix}\text{h}\times\frac{\Big(\frac{\text{N}}{2}-\text{cf}\Big)}{\text{f}}\end{Bmatrix}$
$\Rightarrow24=20+\begin{Bmatrix}10\times\frac{\Big(\frac{55+\text{a}}{2}-30\Big)}{2}\end{Bmatrix}$
$\Rightarrow4=10\times\frac{55+\text{a}-60}{2\text{a}}$
$\Rightarrow4=5\times\frac{\text{a}-5}{\text{a}}$
$\Rightarrow4\text{a}=5\text{a}-25$
$\Rightarrow\text{a}=25$
Thus, yhe missung frequency is 25.

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