The times, t t\,t hours, spent on a task are grouped below.
| Time | 0<t≤40<t\leq40<t≤4 | 4<t≤84<t\leq84<t≤8 | 8<t≤128<t\leq128<t≤12 |
|---|---|---|---|
| Frequency | 4 | aaa | 2 |
Using class midpoints, the estimated mean is given as 5 hours.
Show that a=2a=2a=2.
Calculate an estimate of the variance.