21X/90+125/100(21x/90)+285
7x/30+5/4(7x/30)+285=x
4*7x+5*7x+120*285=120x
120*285=120x-35x-28x
120*285=57x
x=(120*285)/57
x=600 km
Verificare
prima zi 21/90 din 600= 7/30*600=140 km
zi a 3 a 285 km
zi a 2 a 600-140-285=175km sau 125/100(21x/90)⇒ 5/4*7x/30 dar x=600
5/4*140=1,25*140=175 km
total
140+475+285=600
600=600