Solve the unknowns of 140a-50b-30c=80...eqn1). -50a+100b-40c=70...eqn2).
-30a-40b+90c=0...eqn3).

The first one, i believe, would be correct. all the others have one thing in common, and that's that they have repeating numbers. in the second one, x and y have repeating 6's and 3's, and in the third, x and y have repeating 6's as well. in the last one, there are repeating 3's. the first one has no repeating numbers.