1 63 4-: 2 4

\[4 \frac{1}{2} : 6 \frac{3}{4} = \frac{4 \cdot 2 + 1}{2} : \frac{6 \cdot 4 + 3}{4} = \frac{9}{2} : \frac{27}{4} = \frac{9}{2} \cdot \frac{4}{27} = \frac{36}{54} = \frac{2}{3}\]