$17\mathrm{.}4-4.$ A three$-$server queueing system has a controlled arrival

process that provides customers in time to keep the servers continuously busy. Service times have an exponential distribution with

mean $0\mathrm{.}5.$

You observe the queueing system starting up with all three

servers beginning service at time t $=0.$ You then note that the first

completion occurs at time t $=1.$ Given this information, determine

the expected amount of time after t $=1$ until the next service completion occurs