Solution
= Solution
For a renewal holding time $T$ with mean $m$, the <equilibrium residual-life distribution> has survivor $m^{-1}\int_t^\infty\mathbb P(T>s)\,ds$. For C, and also the common long-run B/C renewal structure, $m=2/r$ and the Erlang survivor give
$$
\boxed{\mathbb P(D_{\rm eq}>t)=e^{-rt}\left(1+\frac{rt}2\right),\qquad
f_{D_{\rm eq}}(t)=\frac r2e^{-rt}(1+rt).}
$$
For A the exponential law is already its own equilibrium delay.