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Daca a+b = 15 si 2b + 3c = 27,atunci 2a + 5b + 9c este egal cu......?

Răspuns :

a+b=15; b+3c=27
2a+5b+9c=2a+2b+3b+9c=2×(a+b) + 3×(b+3c)=2×15+3×27=30+81=111