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m in (-oo:+oo)
(4/5)*m+1/10 = 21/50 // - 21/50
(4/5)*m+1/10-(21/50) = 0
(4/5)*m+1/10-21/50 = 0
4/5*m-8/25 = 0 // + 8/25
4/5*m = 8/25 // : 4/5
m = 8/25/4/5
m = 2/5
m = 2/5
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