Customers come to a teller's window according to a Poisson process with a rate of $10$ customers every $25$ minutes. Service times are Exponential. The average service takes $2$ minutes. Compute

$($a$)$ the average number of customers in the system and the average number of customers waiting in a line;

$($b$)$ the fraction of time when the teller is busy with a customer;

$($c$)$ the fraction of time when the teller is busy and at least five other customers are waiting in a line.