viteza medie = distanta totala / timpul total
distanta = d
t total = t₁ + t₂
t₁ = timpul in care strabate 1/3 din d cu viteza v₁= 18 km/h = 5 m/s
t₁ = d / (3 x v₁) = d/15 s
t₂ = timpul in care strabate restul de drum = 2/3 din d cu v₂ = 4 m/s
t₂ = 2d/3v₂ = 2d / (3 x 4 ) = 2d /12 = d/6 secunde
t = t₁ +t₂ = d/15 + d/6 = (2d+ 5d) / 30 = 7d / 60
viteza medie = d/ t = d / (7d/30) = 30 / 7 = 4,2857 m/s
