Q) What is the probability in a single server model that at least one customer is in the system?

Q) What is the probability in a single server model that at least one customer is in the system?

• The answer is 60%
• I would like to know why it is 60%. The way I solved was that P0= 1-0.400 = 0.6% but if is a single server model that at least one customer is in the system then I think it might have to be P3 instead of P0. Please clarify this for me and I appreciate it a lot for your help. Thank you!