Question 925498
time/distance=45min/3miles = 15 min to go 1 mi, which is 4 mph 
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Answer to your comment
The formula of course, Speed = dist/time, s = d/t
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In this problem you are dealing with miles per hour, so we need the times in hrs
45 min = .75 hrs
s = 3/.75
s = 4 mph