Climbing time =30÷2.5= 12 (hours)
Downhill time =30÷ 15=2 (hours)
Average speed = (uphill distance+downhill distance) ÷ (uphill time+downhill time)
=(30+30)÷( 12+2)
=60÷ 14
=30/7 (km/h)
Solution 2:
Average downhill speed = 30× 2 ÷ (30 ÷ 2.5+30 ÷15) = 30/7 (km/h).