dane: v₁=60km/h, vśr=v=80km/h, |AB|=s₁=s₂=s
szukane: v₂
vśr = v = Δs/Δt…Δs=s₁+s₂=2s…Δt=t₁+t₂=s/v₁ + s/v₂
v = 2s /[s/v₁ + s/v₂] = 2/[1/v₁ + 1/v₂]
1/v₁ + 1/v₂ = 2/v
1/v₂ = 2/v - 1/v₁ = [2v₁-v]/v₁v
v₂ = v₁v/[2v₁-v] = 6080/[120-80] km/h = 120km/h
Szukana prędkość powrotna wynosi 120 km/h