At what rate should the blades of the offshore turbine rotate if the speed at the tip is to be the same as that of the turbine on land?

The question is incomplete it should be:

Wind turbines designed for offshore installations are much larger than ones designed for use on land. One company makes a turbine for use on land with blades 39 m long that spin at 17 rpm, and a turbine for use offshore with blades that are twice as long.

The answer is t o obtain the same tip speed, the RPM would need to be reduced to 1/2, or 8.5 RPM.