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m in (-oo:+oo)
5*(4-5*m) = -80 // + 80
5*(4-5*m)+80 = 0
5*(4-5*m)+80 = 0
100-25*m = 0
100-25*m = 0
100-25*m = 0 // - 100
-25*m = -100 // : -25
m = -100/(-25)
m = 4
m = 4
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