$4.-$ It is intended to build an access to a subdivision, it is expected that during the day the users between residents and visitors will reach a rate of $2\mathrm{.}1$ users per minute. There are $3$ entrances and the attention and review capacity of the security guards is one user. in $1\mathrm{.}3$ minutes.

a$)$ What is the probability that there are no users in the system?

b$)$ What is the average time that a user waits before being served?

c$)$ What is the probability that a user will have to wait?

d$)$ If it is intended that no user has to wait more than $2$ minutes from the moment they arrive until they access the subdivision, how many entrances should access have?