Consider a queueing system with two servers, where the customers arrive

from two different sources. From source $1,$ the customers always arrive $2$ at a

time $($although they then will be served one at a time$),$ where the time between

consecutive arrivals of pairs of customers has an exponential distribution with a

mean of $20$ minutes. Source $2$ is itself a two$-$server queueing system, which has a

Poisson input process with a mean rate of $7$ customers per hour, and