The first part is 1/3rd of the total distance and the second part is 2/3rd of the total distance. He travels at s km/hr speed during the first half and 2s km/hr speed during the second half.
If 3 km is the total distance, then 1 km was traveled at s km/hr and 2 kms was traveled at 2s km/hr speed.
Hence average speed = total distance/total time=3/(1/s)+(2/2s)=3/(4/2s)=3s/2. This, however, = s+2s/2=3s/2 which is the arithmetic mean of the speeds of the two parts.
If your mind can conceive it and your heart can believe it, have faith that you can achieve it.