The mean of the following distribution is 18. Find the frequency f of class 19 – 21.
| Class | 11-13 | 13-15 | 15-17 | 17-19 | 19-21 | 21-23 | 23-25 |
| Frequency | 3 | 6 | 9 | 13 | f | 5 | 4 |
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Solution
| Class | Frequency fi | Mid value xi | `f_ix_i` |
| 11-13 | 3 | 12 | 36 |
| 13-15 | 6 | 14 | 84 |
| 15-17 | 9 | 16 | 144 |
| 17-19 | 13 | 18 | 234 |
| 19-21 | f | 20 | 20f |
| 21-23 | 5 | 22 | 110 |
| 23-25 | 4 | 24 | 96 |
Mean = `(sumf_ix_i)/(sum f_i) = (704 + 20f)/(40 + f) = 18 `
`=> 704 + 20f = 720 + 18f`
`=> 2f = 16`
`=> f = 8`
Concept: Mean of Grouped Data
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