2 answers
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Let x = margin width.
(18 - 2x)(22 - 2x) = 96
396 - 80x +4x^2 = 96
300 -80x +4x^2 = 0
x^2 -20x + 75 = 0
(x-15)(x-5) = 0
x = 5 cm is the only margin width that results in positive length and width of the printed area.