\[\boxed{\text{5.360.}}\]
\[\textbf{а)}\ \frac{45}{100} = \frac{5 \cdot 9}{5 \cdot 20} = \frac{9}{20}\]
\[\textbf{б)}\ \frac{75}{1\ 000} = \frac{25 \cdot 3}{25 \cdot 40} = \frac{3}{40}\]
\[\textbf{в)}\ \frac{1\ 125}{1\ 500} = \frac{125 \cdot 9}{125 \cdot 12} = \frac{9}{12} = \frac{3}{4}\text{\ \ }\]