Răspuns:
Explicație pas cu pas:
a = lungimea traseului
prima etapa = 40a/100 = 2a/5
a doua etapa = 2a/5*90/100 = 2a/5*9/10 = 18a/50 = 9a/25
in cele doua etape 2a/5 + 9a/25 = 10a/25 + 9a/25 = 19a/25
etapa a treia a - 19a/25 = 25a/25 - 19a/25 = 6a/25
6a/25 = 9a/25*60/100 + 12
6a/25 = 9a/25*3/5 + 12
6a/25 = 27a/125 + 12
6a/25 - 27a/125 = 12
30a/125 - 27a/125 = 12
3a/125 = 12
a = 125*12/3 = 125*4 = 500 km