Can anybody help with this question? :

A photographer works part-time in a shopping center, two hours per day, four days per week. On the average, six customers arrive each hour, and the arrivals appear to occur independently of one another. Twenty minutes after she arrives one day, the photographer wonders what the chances are that exactly one customer will arrive during the next 15 minutes. Find this probability: (i) if two customers just arrived within the first 20 minutes and (ii) if no customers have come into the shop yet on this particular day.