To calculate the average, the total amount must be divided by the corresponding number of shares, not the sum of the two divided by 2.

Suppose this mountain road is 1 share.

The climbing time is 1÷3= 1/3 (hours).

Downhill time is 1÷5= 1/5 (hours).

The average speed is 2 ÷ (1/3+1/5) = 3.75 (km/h).

Answer: This is for climbers. The average downhill speed is 3.75 km/h.