Question

The mean of the following distribution in 52 and the frequency of class interval 30-40 'f' find f 

C.I	10-20	20-30	30-40	40-50	50-60	60-70	70-80
freq	5	3	f	7	2	6	13

 

Answer 1

C.I	Frequency (f)	Mid value (x)	fx
10-20	5	15	75
20-30	3	25	75
30-40	f	35	35f
40-50	7	45	315
50-60	2	55	110
60-70	6	65	390
70-80	13	75	975
Total	36+f		1940+35f

Mean= ${\displaystyle \frac{\sum fx}{\sum f}=\frac{1940+35f}{36+f}}$ .....(1) 

But, mean =52 .........(2) 

From (1) and (2) 

${\displaystyle \frac{1940+35f}{36+f}=52}$ 

1940 + 35f = 1872 + 52f 

17f = 68 

f = 4