To find the average speed you must know the total distance traveled and the total elapsed. The distance traveled in each interval is simply the speed multiplied by the time (v x t). Thus, in the first interval you travel 120 x 1 = 120 meters, in the second interval 120 x 6 = 720 meters; therefore, the total distance traveled is 840 meters. The total time elapsed is 120 + 120 = 240 seconds. An average speed is simply the total distance traveled divided by the total time elapsed thus in this case it is 720 m / 240 s = 3 m/s.