jmc

A 10-by-12-inch paper will have an area of $10\cdot 12=120$ square inches. If that paper has 1.5 inch margins on all sides, then the portion of the paper that is not covered by margins will be a rectangular section with length $12-2(1.5)=9$ inches and width $10-2(1.5)=7$ inches. Thus, $9\cdot 7=63$ square inches of the paper will not be taken up by the margins. Since the paper has a total area of $120$ square in, this means that $120-63=57$ square inches of the page is taken up by margins. Thus, margins take up $\frac{57}{120}=\boxed{\frac{19}{40}}$ of the page.