Average grain diameter at 310MPa from 2 known yield/grain values

[physicsnnewbie]

Homework Statement

The lower yield point for an iron that has an average grain diameter of 1e-2mm is 230MPa. At a grain diameter of 6e-3mm, the yield point increases to 275MPa. At what grain diameter will the lower yield point be 310MPa?

Homework Equations

Not sure

The Attempt at a Solution

Not sure how to do it.

 

[Mapes]

Look up "Hall-Petch."

 

[physicsnnewbie]

Thanks Mapes