a-caiet
b- radiera
c-creion
a+b+c=7
a+3b+3c=13
2a+2b+c=13
a+b+c=7=>b+c=7-a
a+3b+3c=13
a+3(b+c)=13
daca b+c=7-a din ultimele doua=>
a+3×(7-a)=13
a+21-3a=13
-2a=13-21
-2a=-8=>a=(-8):(-2)=>a=4
b+c=7-a si
a=4=>b+c=7-4=>
b+c=3=>c=3-b
2a+2b+c=13
a=4
c=3-b
2×4+2b+3-b=13
8+b+3=13
11+b=13
b=13-11
b=2
c=3-b
c=3-2
c=1