exam Maths Puzzle Numerical Ability

This question needs to be solved with the addition/elimination method.
0.4x-0.1y=0.3
-80x+20y=40

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exam Other Question

If a bridge that is one mile long is attached to the ground on either side, and you cut it in the middle and add one foot, what is the height of how far the bridge bows up in the middle? please help me figure out this problem. I think I know the answer but I cant figure out how to get there. [3x-2y]^2. I can only come up with 9x^2+4y^2. please help.