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30 + 3 x [ 85 - ( 75 - 65 ) x a + 80 ] = 519

Răspuns :

3x[8[85-(75-65)x a+80]=519-30
3x[85-(75-65)x a+80]=489
[85-(75-65)x a+80]=489:3
[85-(75-65)x a+80]=163
85-(75-65)x a=163-80
85-(75-65)x a=83
(75-65)x a=85-83
10a=2, a=2/10=1/5