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k in (-oo:+oo)
2/5-(2*k) = k-(7/20) // - k-(7/20)
2/5-(2*k)-k+7/20 = 0
2/5-2*k-k+7/20 = 0
3/4-3*k = 0 // - 3/4
-3*k = -3/4 // : -3
k = -3/4/(-3)
k = 1/4
k = 1/4
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