Class interval: | 10-30 | 30-50 | 50-70 | 70-90 | 90-110 | 110-130 |
Frequency: | 5 | 8 | 12 | 20 | 3 | 2 |
Let the assumed mean (A) = 60
Class interval | Mid value (xi) | di = xi - 60 | ui = | Frequency (fi) | fiui |
10-30 | 20 | -40 | -2 | 5 | -10 |
30-50 | 40 | -20 | -1 | 8 | -8 |
50-70 | 60 | 0 | 0 | 12 | 0 |
70-90 | 80 | 20 | 1 | 20 | 20 |
90-110 | 100 | 40 | 2 | 3 | 6 |
110-130 | 120 | 60 | 3 | 2 | 6 |
N = 50 | ∑fiui = 14 |
We have, A = 60
h = 20
Mean = A + h * 
= 60 + 20 * 
= 60 + 5.6
= 65.6
12