The times, t t\,t minutes, taken by 80 drivers to complete a journey are summarised in the table below.
| Time (minutes) | Frequency |
|---|---|
| 0<t≤100 < t \le 100<t≤10 | 6 |
| 10<t≤2010 < t \le 2010<t≤20 | 18 |
| 20<t≤3020 < t \le 3020<t≤30 | 30 |
| 30<t≤5030 < t \le 5030<t≤50 | 20 |
| 50<t≤8050 < t \le 8050<t≤80 | 6 |
Estimate the mean time taken.
Estimate the standard deviation of the times.