Because the length of the mountain road is S.

The round trip is 2S.

The uphill time is S/V 1, and the downhill time is S/V2.

Average speed = total distance/total time =2S/ (sec /v 1+ sec /V2)

= 2S/[(S(v 1+V2)/v 1 * V2]

=(2V 1V2)/(V 1+V2)

Note: (V 1+V2)/2 cannot be used to find the average speed.