Answer
See below.
Work Step by Step
Plugging in $t=4$ we get: $s(4)=\frac{60}{4}=15$, this means that if it takes $4$ minutes to run a mile, her average speed will be $15$ mph.
Plugging in $t=5$ we get: $s(5)=\frac{60}{5}=12$, this means that if it takes $5$ minutes to run a mile, her average speed will be $12$ mph.