d₁=d/3
d₂+d₃=d-d₁=d-d/3=2d/3
d₂=2d/3/2=2d/6=d/3
d₃=2d/3-d₂=2d/3-d/3=d/3
t=t₁+t₂+t₃
t₁=d₁/v₁=d/3/v₁=d/3v₁
t₂=d₂/v₂=d/3/v₂=d/3v₂
t₃=d₃/v₃=d/3/v₃=d/3v₃
d=vm*t=>vm=d/(t₁+t₂+t₃)=d/(d/3v₁+d/3v₂+d/3v₃)=d/d(1/3v₁+1/3v₂+1/3v₃)=1/(v₂v₃+v₁v₃+v₁v₂)/v₁v₂v₂=v₁v₂v₂/(v₂v₃+v₁v₃+v₁v₂)