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x in (-oo:+oo)
80 = (2/5)*x // - (2/5)*x
80-((2/5)*x) = 0
(-2/5)*x+80 = 0
80-2/5*x = 0 // - 80
-2/5*x = -80 // : -2/5
x = -80/(-2/5)
x = 200
x = 200
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