4. A company offering a bicycle courier service within London collected data on the delivery times for a sample of jobs completed by staff at each of its two offices.
The times, \(t\) minutes, for 20 deliveries handled by the company's Hammersmith office were summarised by
$$\Sigma t = 427 , \text { and } \Sigma t ^ { 2 } = 11077$$
- Find the mean and variance of the delivery times in this sample.
The company's Holborn office handles more business, so the delivery times for a sample of 30 jobs handled by this office was taken. The mean and standard deviation of this sample were 18.5 minutes and 8.2 minutes respectively.
- Find the mean and variance of the delivery times of the combined sample of 50 deliveries.