Two sets of vehicles are observed over a kilometer of road, and their flows are also recorded. In the first set, four vehicles take 52, 56, 63 and 69 seconds when the flow is 1,500 veh/hr. In the second set, four vehicles take 70, 74, 77 and 79 seconds when the flow is 1920 veh/hr. These vehicles have been timed over space, so the space mean speed can be calculated from the average times for each set. Calculate the space-mean speed,