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

Class Interval Mid Value (xi) Frequency (fi) fixi
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
Σfixi = 704 + 20f
Given : Mean = 18
We know, Mean (x̄) = Σfixi / Σf
⇒ 18 = (704 + 20f) / (40 + f)
⇒ 18(40 + f) = 704 + 20f
⇒ 720 + 18f = 704 + 20f
⇒ 720 - 704 = 20f - 18f
⇒ 16 = 2f
⇒ f = 8