Class interval: | 0-6 | 6-12 | 12-18 | 18-24 | 24-30 |
Frequency: | 7 | 5 | 10 | 12 | 6 |
Let the assumed mean (A) = 15
Class interval | Mid value (xi) | di = xi - 15 | ui = | Frequency (fi) | fiui |
3 – 6 | 3 | -12 | -2 | 7 | -14 |
6 – 12 | 9 | -6 | -1 | 5 | -5 |
12 – 18 | 15 | 0 | 0 | 10 | 0 |
18 – 24 | 21 | 6 | 1 | 12 | 12 |
24 - 30 | 27 | 12 | 2 | 6 | 12 |
N = 40 | ∑fiui = 5 |
We have, A = 15
h = 6
Mean = A + h * 
= 15 + 6 * 
= 15 + 0.75 = 15.75
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