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Grazes
Each tire on a car has a radius of 15 inches. The tires are rotating at 450 revolutions per minute. Find the speed of the automobile to the nearest mile per hour.
Let's look at this in steps: We know the radius of the tire, and the number of rotations per minute. We'd like to know the distance per rotation so we can use rotations per time to translate into miles per hour. So: \[C = 2\pi r\] With this, we can solve for distance per rotation. We also know that: \[Distance = C * rpm\] And once we have the final distance, just divide by 60 to change from distance per minute to distance per hour (and convert from inches per hour to miles per hour).
Don't you multiply by 60?