x-mere
y-portocale
z-mandarine
x+y+z=879
x=12+2y si x=z-15 atunci
12+2y=z-15
z=2y+12+15
z=2y+27
avem x=12+2y,z=2y+27 inlocuim in prima ecuatie
12+2y+y+2y+27=879
5y=879-39
5y=840
y=168kg-cantitatea de portocale
x=12+2*168=348
x=348kg-cantitatea de mere
z=2*168+27=336+27=363
z=363kg -cantitatea de mandarine
348+168+363=879(proba)