The lower yield point for an iron that has an

The lower yield point for an iron that has an average grain diameter of 5 ( 10-2 mm is 135MPa (19,500psi). At a grain diameter of 8 ( 10-3 mm, the yield point increases to 260MPa (37,500psi). At what grain diameter will the lower yield point be 205MPa (30,000psi)?