$2.$ According to SSUN, an email user gets on average $138$ emails per day. Let X$-$the number of emails an email user receives per day. The discrete random variable X takes on the values x$=0,1,2.....$ The random variable X has a Poisson distribution: X$-$P$(138).$ The mean is $138$ emails.

a$)$ What is the probability that an email user receives exactly $160$ emails per day?

b$)$ What is the probability that an email user receives at most $160$ emails per day?