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a in (-oo:+oo)
4 = (2/5)*a // - (2/5)*a
4-((2/5)*a) = 0
(-2/5)*a+4 = 0
4-2/5*a = 0 // - 4
-2/5*a = -4 // : -2/5
a = -4/(-2/5)
a = 10
a = 10
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