a+b+c+d=30 a+d=10 7a+9b+9xc+7d

Răspuns :

a+b+c+d=30  ; a+d = 10  => b+c = 30-10 = 20

7a+9b+9c+7d = 7·(a+d)+9·(b+c) = 7·10 +9·20 = 70+180 = 250

7a+9b+9c+7d rezulta a+b+c=30 rezulta a+d=10 rezulta b+c=20
7a+7d+9b+9c=
7(a+d)+9(b+c)=
7·10+9·20=
70+180=
250