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r in (-oo:+oo)
(-4/5)*r-6 = -30 // + 30
(-4/5)*r-6+30 = 0
24-4/5*r = 0 // - 24
-4/5*r = -24 // : -4/5
r = -24/(-4/5)
r = 30
r = 30
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