\[\boxed{\text{5.387.}}\]
\[\textbf{а)}\ 28\ :7 = 4\]
\[\frac{5}{7} = \frac{5 \cdot 4}{7 \cdot 4} = \frac{20}{28};\ \]
\[\textbf{б)}\ 60\ :15 = 4\]
\[\frac{11}{15} = \frac{11 \cdot 4}{15 \cdot 4} = \frac{44}{60};\ \]
\[\textbf{в)}\ 76\ :19 = 4\]
\[\frac{13}{19} = \frac{13 \cdot 4}{19 \cdot 4} = \frac{52}{76};\ \]
\[\textbf{г)}\ 75\ :15 = 5\]
\[\frac{11}{15} = \frac{11 \cdot 5}{15 \cdot 5} = \frac{55}{75}\text{.\ \ }\]