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b in (-oo:+oo)
(4/5)*b+4 = 21/5 // - 21/5
(4/5)*b-(21/5)+4 = 0
(4/5)*b-21/5+4 = 0
4/5*b-1/5 = 0 // + 1/5
4/5*b = 1/5 // : 4/5
b = 1/5/4/5
b = 1/4
b = 1/4
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