The times, t t\,t minutes, for 50 customers to be served 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 | 12<t≤1612<t\leq1612<t≤16 |
|---|---|---|---|---|
| Frequency | 10 | 20 | aaa | 5 |
Using class midpoints, the estimated mean is 7.2 minutes.
Show that a=15a=15a=15.
Calculate an estimate of the variance.