First, note that
[tex]\dfrac{4}{5}=\dfrac{4\cdot 2}{5\cdot 2}=\dfrac{8}{10}.[/tex]
She counts the mileage markers that are placed every [tex]\dfrac{1}{10}[/tex] of a mile along the trail. Then she counts
[tex]\dfrac{8}{10}:\dfrac{1}{10}=\dfrac{8}{10}\cdot \dfrac{10}{1}=\dfrac{8\cdot 10}{10\cdot 1}=\dfrac{8}{1}=8[/tex] markers.
Answer: 8 markers (if first marker is on the start of trail).