To find Albert's unit rate in miles per hour, we need to divide the distance he walked by the time it took. The correct expression to calculate the unit rate in miles per hour would be:
\[ \frac{4/5}{5/12} \]
This can also be written as:
\[ 4/5 \div 5/12 \]
Thus, the expression that can be used to calculate his unit rate in miles per hour is:
\[ \frac{4}{5} \div \frac{5}{12} \]
Therefore, from the options provided, the correct choice is:
4 / 5 / 5 / 12 (interpreted as \(\frac{4}{5} \div \frac{5}{12}\))