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a in (-oo:+oo)
7/8-((4/5)*a)-(5*a) = -661/40 // + -661/40
7/8-((4/5)*a)-(5*a)-(-661/40) = 0
(-4/5)*a-5*a+7/8+661/40 = 0
87/5-29/5*a = 0 // - 87/5
-29/5*a = -87/5 // : -29/5
a = -87/5/(-29/5)
a = 3
a = 3
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