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A in (-oo:+oo)
3/10 = A/40 // - A/40
3/10-(A/40) = 0
3/10-1/40*A = 0 // - 3/10
-1/40*A = -3/10 // : -1/40
A = -3/10/(-1/40)
A = 12
A = 12
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