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x in (-oo:+oo)
(1/4)*x-((5/8)*x) = 15-30 // - 15-30
(1/4)*x-((5/8)*x)-15+30 = 0
(1/4)*x+(-5/8)*x-15+30 = 0
15-3/8*x = 0 // - 15
-3/8*x = -15 // : -3/8
x = -15/(-3/8)
x = 40
x = 40
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