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Explanation:
Subtract the mile marker values to find the distance traveled. Think of it like markers on a number line. Though in reality the road is likely curved in many ways.
Going from marker 30 to marker 120 is a distance of 120-30 = 90 miles.
This happens in 5-4 = 1 hour, so,
speed = distance/time = 90/1 = 90 miles per hour
