The table header tells you the distances are <em>feet</em> (not yards). The maximum height is 55 feet, and the lowest point is 5 feet. The difference between these values is the diameter of the wheel, 50 feet, so the radius is 25 feet.
The wheel goes from its highest point to its highest point in 15 -3 = 12 seconds, so that is the time required for a complete revolution.
