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a+ b = 20
b + c = 30
3 x a + 7 x b + 4 X c = ?


Răspuns :

(a+b=20)*3=3a+3b=60

(b+c=30)*4=4b+4c=120

---------------------------------------- +

3a+3b+4b+4c=60+120

3a+7b+4c=180

R: 180