I've been working on this problem for quite a long time now but I'm not being able to solve it. It goes as follows : A coffee shop opens his doors from 9.00 to 1.pm. supposedly 8 customers will enter this shop and stay for a period of half an hour. Let X be the number of customers present in this shop at a certain time t, what is the probabilty mass function of X ? I would be very much grateful if you could please help me in solving this problem. Thank you.