The Total solution for NCERT class 6-12
The following distribution shows the daily pocket allowance of childrenof a locality. The mean pocket allowance is Rs 18. Find the missing frequencyf.
Daily Pocket Allowance(in c)
11-13
13-15
15-17
17-19
19-21
21-23
23-35
Number of children
7
6
9
13
f
5
4
Solution:
To find out themissing frequency, use the mean formula.
Here, the value ofmid-point (xi) mean x̄ = 18
Class interval
Number of children (fi)
Mid-point (xi)
fixi
12
84
14
16
144
18 = A
234
20
20f
22
110
23-25
24
96
Total
fi = 44+f
Sum fixi = 752+20f
The mean formula is
Mean = x̄ = ∑fixi /∑fi =(752+20f)/(44+f)
Now substitute thevalues and equate to find the missing frequency (f)
⇒ 18 = (752+20f)/(44+f)
⇒ 18(44+f) = (752+20f)
⇒ 792+18f = 752+20f
⇒ 792 – 752 = 20f – 18f
⇒ 40 = 2f
⇒ f = 20
So, the missingfrequency, f = 20.