x- numarul total de caiete
1 sapt: x-x/3-80=A unde A reprezinta restul de caiete ramase
2 sapt: A-2A/3+60=B unde B reprezinta noul numar de caiete ramase
3sapt: B-B/2-60=0
facem drumul invers
B-B/2-60=0
B-B/2=60 aducem la acelasi numitor comun
2B-B=120
B=120
A-2A/3+60=120
A-2A/3=120-60
A-2A/3=60 amplificam cu 3
3A-2A=180
A=180
x-x/3-80=180
x-x/3=180+80
x-x/3=260 amplificam cu 3
3-x=780
2x=780
x=390