$7\mathrm{.}24$ The lower yield point for an iron that has an av$-$

erage grain diameter of $5{10}^{-2}mm$ is $135$ MPa

$(19,500).$ At a grain diameter of $8{10}^{-3}mm,$

the yield point increases to $).$

At what grain diameter will the lower yield point

be $205$MPa$(30,000)?$