Answer
$35~mi/h$
Work Step by Step
$15~min=\frac{1}{4}~h=0.25~h$
The first 15 minutes:
$distance=(average~speed)(time~elapsed)=(5~mi/h)(0.25~h)=1.25~mi.$
Now, I need to travel 8.75 miles, in the 15 minutes (0.25 h) remaining:
$average~speed=\frac{distance}{time~elapsed}=\frac{8.75~mi}{0.25~h}=35~mi/h.$
