At Metropolis City Hall, two workers "pull strings" (make deals) every day. Strings arrive to be pulled on an average of one every 10 minutes throughout the day. It takes an average of 15 minutes to pull a string. both times between arrivals and service times are exponentially distributed.

A) What is the probability that there are no strings to be pulled in the system at a random point in time?

b) what is the expected number of strings waiting to be pulled?

c) What is the probability that both string-pullers are busy?

d) What is the effect on performance if a third string-puller, working at the same speed as the first two, is added to the system?