It took Mike 5.625 hours to travel over pack ice from one town in the Arctic to another town 180 miles away. During the return journey, it took him 10 hours. Assume the pack ice was drifting at a constant rate, and that Mike's snowmobile was traveling at a constant speed relative to the pack ice. What was the speed of Mike's snowmobile?:
a. 25 miles per hour
b. 7 miles per hour
c. 22 miles per hour
d. 13 miles per hour