@nntaleb travels:
10 km at 10 km/h
10 km back at 5 km/h
Total dist = 20 km
Going:
Distance = 10 km
Speed = 10 km/h
Time = D / S = 10/10 = 1 hr
Returning:
D = 10 km
S = 5 km/h
T = D / S = 10/5 = 2 hrs
Total T = 1 + 2 = 3 hours
Average Speed = Total D / Total T = 20 ÷ 3 = 6.67 km/h