Hello, Not sure how to go about solving this problem: What outer diameter must a 2mm thick hollow rod have in order to have the same structural bending strength as a solid rod with diameter of 7mm? Also compare the weight per unit length of the two rods. Assume that material is stainless steel (E=180MPa) having weight density of 7.78 x 10^4N/m^3. Does the 2mm thick refer to the length of the hollow rod? Where does weight density play into the equation? Given equations: stress = Mc/I Bending rigidity -> ymax= cFl^3/EI Thank you in advance for any information you can provide.