What is the average speed if you run a kilometer at 5 m/s and then walk a kilometer at 1 m/s ? (Note: 3 m/s is not correct.)

Average Speed is the distance traveled divided by the time involved. So in this
case the distance was 2 km which equals 2000 m. The total time will be the sum 
of the times spent running and walking:

                     distance     1 km      1000 m
     Time Running = ---------- = ------- = -------- = 200 s
                       speed      5 m/s     5 m/s


                     distance     1 km      1000 m
     Time Walking = ---------- = ------- = -------- = 1000 s
                       speed      1 m/s     1 m/s


     Total Time = 200 s  +  1000 s  =  1200 s


Therefore,

                      distance     2000 m     
     Average speed = ---------- = -------- = 1.67 m/s
                        time       1200 s