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x in (-oo:+oo)
4/12 = x/15 // - x/15
4/12-(x/15) = 0
1/3-1/15*x = 0 // - 1/3
-1/15*x = -1/3 // : -1/15
x = -1/3/(-1/15)
x = 5
x = 5
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