ANSWER:
40%
STEP-BY-STEP EXPLANATION:
In this case, what we must do is calculate the percentage that represents 21 miles, assuming that 100% is 15 miles, like this
[tex]21\cdot\frac{100}{15}=140\text{\%}[/tex]Now we subtract 100% from this value, like this:
[tex]140\text{\%}-100\text{\%}=40\text{\%}[/tex]