51:1= 8 4

$$ 5 \frac{1}{8} : \frac{1}{4} = \frac{5 \cdot 8 + 1}{8} : \frac{1}{4} = \frac{41}{8} : \frac{1}{4} = \frac{41}{8} \cdot \frac{4}{1} = \frac{41 \cdot 4}{8 \cdot 1} = \frac{41 \cdot 1}{2 \cdot 1} = \frac{41}{2} = 20 \frac{1}{2} $$

Ответ: $$20 \frac{1}{2}$$