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June 2005 p6 q2
2445
The following table shows the results of a survey to find the average daily time, in minutes, that a group of schoolchildren spent in internet chat rooms.
| Time per day (t minutes) | Frequency |
|---|---|
| \(0 \leq t < 10\) | 2 |
| \(10 \leq t < 20\) | f |
| \(20 \leq t < 40\) | 11 |
| \(40 \leq t < 80\) | 4 |
The mean time was calculated to be 27.5 minutes.
- Form an equation involving \(f\) and hence show that the total number of children in the survey was 26. [4]
- Find the standard deviation of these times. [2]
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