It is known that

$$ 1\ hour=60\ minutes $$ It is given that Katerina runs 15 miles in $2\frac{1}{2}$ hours, thus she runs 1 mile in

$$ \frac{2\frac{1}{2}}{15} \ hours $$

To convert hours into minutes, we multiple by 60:

$$ \frac{2\frac{1}{2}}{15}\times60 =10\ minutes $$