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b in (-oo:+oo)
257/105 = 5*b+(1/3)*b-(3/5) // - 5*b+(1/3)*b-(3/5)
257/105-(5*b)-((1/3)*b)+3/5 = 0
(-1/3)*b-5*b+257/105+3/5 = 0
64/21-16/3*b = 0 // - 64/21
-16/3*b = -64/21 // : -16/3
b = -64/21/(-16/3)
b = 4/7
b = 4/7
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