x-distanta parcursa de biciclist in primele doua ore;
t-lungimea drumului;
x=7+(t-x)/4⇒x=(28+t-x)/4⇒4x=28+t-x⇒28+t=4x-x⇒28+t=3x⇒x=(28+t)/3;
t=x+20;
t=(28+t)/3+20
3t=28+t+60
3t=t+88
3t-t=88
2t=88
t=88/2
t=44km este lungimea drumului;
Verificam:
x=(28+44)/3=24 km a parcurs in primele doua ore;
24+20=44km.
Bafta!