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x in (-oo:+oo)
x = (400-(3*y))/10 // - (400-(3*y))/10
x-((400-(3*y))/10) = 0
x-((400-3*y)/10) = 0
(-1*(400-3*y))/10+x = 0
(-1*(400-3*y))/10+(10*x)/10 = 0
10*x-1*(400-3*y) = 0
10*x+3*y-400 = 0
(10*x+3*y-400)/10 = 0
(10*x+3*y-400)/10 = 0 // * 10
10*x+3*y-400 = 0
10*x+3*y-400 = 0 // - 3*y-400
10*x = -(3*y-400) // : 10
x = (-(3*y-400))/10
x = (400-(3*y))/10
x = (400-(3*y))/10
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