$4.$ Consider a bus route with two buses, each of which transports passengers from a downtown depot to various destinations and then returns to the depot for another trip. The round trip time is very nearly equal to $\backslash ($ R $\backslash ).$ However, the buses are operated by independent drivers who make no attempt to coordinate their schedules. One of the buses runs slightly faster than the other so that, over many trips, the fraction of trips that the second bus departs $($or arrives$)$ within some time $\backslash ($ t $\backslash )$ after the first bus is $\backslash ($ t $/$ R$,0$