Consider a $n$ server parallel queueing system, need to calculate the probability of $1$ busy server as seen by next arrival process.
$\lambda$$=$$arrival$ $rate$ $of$ $processes$ ; $\mu$$=$$service$ $rate$ $of$ $processes$
When there are $0$ servers busy then next arrival will find $0$ busy servers for sure. If there is currently $1$ busy server, the next arrival finds $1$ busy server if when the time to the next arrival is less than the remaining service time, this is given as $\lambda/(\lambda+\mu)$. Can any one please explain how it is $\lambda/(\lambda+\mu)$, please provide me the material so I can understand such basic concepts better.