A unit rate is a ratio. Here, the rate of interest is Harold's speed, found by dividing the distance Harold walked by the time it took him to do it.
[tex]\text{speed}=\dfrac{\text{distance}}{\text{time}}=\displaystyle\frac{\left(\frac{3}{4}\right)\,\text{miles}}{\left(\frac{5}{6}\right)\,\text{hours}}\\\\=\frac{3}{4}\cdot\frac{6}{5}\,\frac{\text{miles}}{\text{hour}}=\frac{3\cdot 6}{4\cdot 5}\,\frac{\text{miles}}{\text{hour}}\\\\=\frac{9}{10}\,\frac{\text{miles}}{\text{hour}}[/tex]
