R$18.$ How long does it take a packet of length $1,000$ bytes to propagate over a link of distance $2,500km,$ propagation speed $2\mathrm{.}5*{10}^8\frac{m}{s},$ and transmission rate $2$ Mbps$?$ More generally, how long does it take a packet of length $L$ to propagate over a link of distance $d,$ propagation speed $s,$ and transmission