They mean 1.5x the radius.
Since the acceleration due to gravity is proportional to r^-2, this results in a value of g of 4g[earth]/9. But it is also proportional to the mass, in this case estimated to be 5 earth masses. So we get a final value for the acceleration due to gravity of 20*g[earth]/9, or about 21.8 ms^-2.
This also answers Cowperson's assertion. How fast an object is spinning is inconsequential.
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