Question

Find the missing frequency form the following data, if athematic mean is 25.4.

Class-interval	10−20	20−30	30−40	40−50	50−60
Frequency	20	15	10	?	2

Answer 1

Class Interval	Mid-Values (m)	Frequency (f)	fm
10 − 20 20 − 30 30 − 40 40 − 50 50 − 60	15 25 35 45 55	20 15 10 f 2	300 375 350 45f 110
		$\mathit{\Sigma }\mathit{f}$=47+f	$\mathit{\Sigma }\mathit{f}\mathit{m}$=1135 + 45 f

Let missing frequency be f

Given:

Mean, $\overline{\mathit{X}}$ =25.4

$$\begin{gathered} \overline{X}=\frac{\Sigma fm}{\Sigma f} \\ or,25.4=\frac{1135+45f}{47+f} \\ or,1193.8+25.4f=1135+45f \\ or,58.8=19.6f \\ \Rightarrow f=3 \end{gathered}$$

Hence, missing frequency is 3