A fast food restaurant recorded a mean customer arrival of 100 people per hour. There are four (4) employees on each shift. Each employee will take, on average, two (2) minutes to serve a customer, with service time scattered exponentially. Customers who arrive while all staff are busy will line up in a line. 

a) Determine the mean number of employees who are busy. 

b) Find the percentage of the number of employees who are busy. 

c) Calculate the percentage of customers who do not have to line up when arriving at the restaurant.