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(2/3)*x+y = 400 // - 400
(2/3)*x+y-400 = 0
2/3*x+y-400 = 0 // - y-400
2/3*x = -(y-400) // : 2/3
x = (-(y-400))/2/3
x = 3/2*(400-y)
x = 3/2*(400-y)
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