To calculate the percent error in Mark's estimate, we can use the following formula:
\[ \text{Percent Error} = \left( \frac{|\text{Estimated Value} - \text{Actual Value}|}{\text{Actual Value}} \right) \times 100 \]
In this case:
- Estimated Value = 15.5 mi
- Actual Value = 14.5 mi
Now, let's compute the absolute error:
\[ |\text{Estimated Value} - \text{Actual Value}| = |15.5 - 14.5| = 1.0 \text{ mi} \]
Now we can calculate the percent error:
\[ \text{Percent Error} = \left( \frac{1.0}{14.5} \right) \times 100 \]
Calculating this:
\[ \frac{1.0}{14.5} \approx 0.0689655172 \]
Now multiply by 100 to convert to a percentage:
\[ 0.0689655172 \times 100 \approx 6.89655172 % \]
Rounding to the nearest tenth of a percent, we get:
\[ \text{Percent Error} \approx 6.9% \]
Thus, the percent error in Mark's estimate is:
\[ \boxed{6.9} \]