Question:
If the median of the following data is 32.5, find the missing frequencies.
Solution:
Given: Median = 32.5
We prepare the cumulative frequency table, as given below.
Now, we have
$N=40$
$31+f_{1}+f_{2}=40$
$f_{2}=9-f_{1}$.....(1)
Also, $\frac{N}{2}=20$
Since median $=32.5$ so the median class is $30-40$.
Here, $l=30, f=12, F=14+f_{1}$ and $h=10$
We know that
Median $=I+\left\{\frac{\frac{N}{2}-F}{f}\right\} \times h$
$32.5=30+\left\{\frac{20-\left(14+f_{1}\right)}{12}\right\} \times 10$
$2.5=\frac{\left(6-f_{1}\right) \times 10}{12}$
$2.5 \times 12=60-10 f_{1}$
$10 f_{1}=60+30$
$f_{1}=\frac{30}{10}$
$=3$
Putting the value of $f_{1}$ in (1), we get
$f_{2}=9-3$
$=6$
Hence, the missing frequencies are 3 and 6.