In a post office, postmen must retrieve their assignments from the office which is managed by an assistant. Suppose the mean number of postmen retrieving their assignments per hour is 4 and they are paid 10 dollars per hour.

There are two possible assistants A or B to be employed for managing the office. On average assistant A takes 11:5 min to handle one request and is paid 9:5 dollars per hour.

While assistant B takes 12:5 min to handle one request and is paid 7 dollars per hour.

Assume that the inter-arrival time of postmen and the processing time of the assistants are exponentially distributed. Which assistant should be employed?