Question

The mean of the following frequency distribution is 18. The frequency f in the class interval 19 – 21 is missing. Determine f.

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

Answer 1

Class	Class mark (x)	Frequency (f)	fx
11 – 13	12	3	36
13 – 15	14	6	84
15 – 17	16	9	144
17 – 19	18	13	234
19 – 21	20	f	20f
21 – 23	22	5	110
23 – 25	24	4	96
		Σf = 40 + f	Σfx = 704 + 20f

For x, Σf = 40 + f

Σfx = 704 + 20f

Mean = 18 = `(704 xx 20f)/(40 + f)`

⇒ 720 + 18f = 704 + 20f

⇒ f = 8