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2014-01-05T15:06:12+01:00
x+13 ≥ 3/4 (x+20)
x+13 
≥ 3/4x+60/4
x-3/4x 
≥ 15-13
4/4x-3/4x ≥ 2
1/4x ≥ 2
x ≥ 2*4
x ≥ 8