а) $$5\frac{1}{4} : \frac{4}{5} = \frac{21}{4} : \frac{4}{5} = \frac{21}{4} \cdot \frac{5}{4} = \frac{21 \cdot 5}{4 \cdot 4} = \frac{105}{16} = 6\frac{9}{16}$$
Ответ: $$6\frac{9}{16}$$