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 photograp}J.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 minut and (ii) if no customers have come into the shop yet on this particular day.