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Daca : a+b+c=198 si
b-c=123
Atunci : 2a+b+3c= ?


Răspuns :

a+b+c=198    /*2
b-c=123
___________
2a+2b+2c=396
b-c=123            _
_____________
2a+2b-b+2c+c=396-123
2a+b+3c=273
2a+2b+2c=2*198=396
si
b-c=123
  

atunci scazand pe adoua din prima obtinem ce ne intereseaza
2a+2b+2c-(b-c)=2a+2b+2c-b+c=2a+b+3c=396-123=273