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D( x )
20-(3*x) = 0
20-(3*x) = 0
20-(3*x) = 0
20-3*x = 0
20-3*x = 0 // - 20
-3*x = -20 // : -3
x = -20/(-3)
x = 20/3
x in (-oo:20/3) U (20/3:+oo)
(40-(5*m))/(20-(3*x)) = 5/1 // - 5/1
(40-(5*m))/(20-(3*x))-(5/1) = 0
(40-5*m)/(20-3*x)-5 = 0
(40-5*m)/(20-3*x)+(-5*(20-3*x))/(20-3*x) = 0
40-5*(20-3*x)-5*m = 0
15*x-5*m-60 = 0
(15*x-5*m-60)/(20-3*x) = 0
(15*x-5*m-60)/(20-3*x) = 0 // * 20-3*x
15*x-5*m-60 = 0
15*x-5*m-60 = 0 // + -5*m-60
15*x = -(-5*m-60) // : 15
x = (-(-5*m-60))/15
x = (60-(-5*m))/15
x = (60-(-5*m))/15
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