Class interval: | 0-6 | 6-12 | 12-18 | 18-24 | 24-30 |
Frequency: | 6 | 8 | 10 | 9 | 7 |
Let A assumed mean be 15.
Class interval | Mid value (xi) | di = xi - 15 | ui = | fi | fiui |
0 – 6 | 3 | -12 | -2 | 6 | -12 |
6 – 12 | 9 | -6 | -1 | 8 | -8 |
12 – 18 | 15 | 0 | 0 | 10 | 0 |
18 – 24 | 21 | 6 | 1 | 9 | 9 |
24 – 30 | 27 | 12 | 2 | 7 | 14 |
N = 40 | ∑fiui = 3 |
A = 15, h = 6
Mean = A + h * 
= 15 + 6 * 
= 15 + 0.45
= 15.45
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