To find Albert's unit rate in miles per hour, we need to divide the distance he walked (in miles) by the time it took him (in hours). Therefore, the expression we need is:
\[ \text{Unit Rate} = \frac{\text{Distance}}{\text{Time}} = \frac{4/5}{5/12} \]
This can also be expressed as multiplying by the reciprocal of the denominator:
\[ \frac{4/5}{5/12} = 4/5 \times \frac{12}{5} \]
Thus, the correct expression to calculate Albert's unit rate in miles per hour is:
\[ \frac{4/5}{5/12} \]
So the answer is:
4/5 / 5/12