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.