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12-((4/5)*(1*x+15)) = 4 // - 4
12-((4/5)*(1*x+15))-4 = 0
(-4/5)*(x+15)-4+12 = 0
12-4/5*(x+15)-4 = 0
12-4/5*(x+15)-4 = 0
12-4/5*x-16 = 0
-4/5*x-4 = 0
-4/5*x-4 = 0
-4/5*x-4 = 0 // + 4
-4/5*x = 4 // : -4/5
x = 4/(-4/5)
x = -5
x = -5
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