If we use unit rates, we can calculate the unit rate for the kilometers per trip as:
![u=\frac{3.2\operatorname{km}}{6\text{ trips}}\approx0.5333\frac{\operatorname{km}}{\text{trip}}]()
Then, for 15 trips, we can calculate the kilometers as:
![D=n\cdot u=15\text{ trips}\cdot0.5333\frac{\operatorname{km}}{\text{trip}}=7.9995\operatorname{km}\approx8\operatorname{km}]()
Answer: After 15 trips, she has walked 8 km.
Alternative approach (not using unit rates):
We can apply the rule of three to solve this problem.
We know that 6 trips correspond to 3.2 km.
Then, 15 trips will correspond to:
![\begin{gathered} 6\text{ trips}\longrightarrow3.2\operatorname{km} \\ 15\text{ trips}\longrightarrow x=15\text{ trips}\cdot\frac{3.2\operatorname{km}}{6\text{ trips}}=8\operatorname{km} \end{gathered}]()