The rate would be 75 miles in 1 hour.
Set up a proportion to solve this:
3 1/8 miles in 2.5 minutes gives us the ratio 3.125/2.5.
The second ratio would be x miles in 60 minutes, or x/60.
3.125/2.5 = x/60
Cross multiply:
187.5 = 2.5x
Divide both sides by 2.5:
187.5/2.5 = 2.5x/2.5
75=x
