Write the Kolmogorov Forward and Backward Equations for Example $11\mathrm{.}5.$

In Example $11\mathrm{.}5$set up the Forward Equations assuming that there are three identical repair crews, each with the same service rate $.$ Each crew services at most one machine at a time and the crews do not cooperate with each other. That is$,$ if i machines are down for repairs, then for i crews stand idle awaiting a new breakdown and $\frac{?}{?}$ crews perform service on the down machines. Assume $N=7.$