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(2/5)*(x+y) = 6 // - 6
(2/5)*(x+y)-6 = 0
2/5*(x+y)-6 = 0
2/5*(x+y)-6 = 0
2/5*x+2/5*y-6 = 0
2/5*x+2/5*y-6 = 0
2/5*x+2/5*y-6 = 0 // - 2/5*y-6
2/5*x = -(2/5*y-6) // : 2/5
x = (-(2/5*y-6))/2/5
x = 5/2*(6-(2/5*y))
x = 5/2*(6-(2/5*y))
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