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This is Giancoli Answers with Mr. Dychko. The total distance this ball travels when it rolls twelve times is 3.5 meters and each revolution of the ball, it will travel a distance of the circumference of the ball. And so the total distance covered then is the number of revolutions times the circumference—number of meters per revolution— and the circumference is π times diameter. So we can solve for diameter by dividing l and this side by number of revolutions times π. So diameter is l over number of revolutions times π so that's 3.5 meters divided by 12.0 and divide by π and that gives 9.3 centimeters must be the diameter of the ball.
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