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y in (-oo:+oo)
(4/5)*y-1 >= 15 // - 15
(4/5)*y-15-1 >= 0
4/5*y-16 >= 0 // + 16
4/5*y >= 16 // : 4/5
y >= 16/4/5
y >= 20
y in <20:+oo)
<20:+oo)
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