Question

The mean of the following distribution is 18. Find the frequency f of class 19 – 21.

Class	11-13	13-15	15-17	17-19	19-21	21-23	23-25
Frequency	3	6	9	13	f	5	4

 

Answer 1

Class	Frequency fi	Mid value xi	`f_ix_i`
11-13	3	12	36
13-15	6	14	84
15-17	9	16	144
17-19	13	18	234
19-21	f	20	20f
21-23	5	22	110
23-25	4	24	96

Mean = `(sumf_ix_i)/(sum f_i) = (704 + 20f)/(40 + f) = 18 `

`=> 704 + 20f = 720 + 18f`

`=> 2f = 16` 

`=> f = 8`