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310-a×(100-30×3)=280

Răspuns :

310-a×(100-30×3)=280
a×(100-90) = 310-280
a×10 = 30
a = 30:10
a = 3
310 - a×(100-30×3)=280

310 - 10a = 280

310 - 280 = 10a

10a = 30

a = 3