Question: The following three sections have the same concrete area $(150,000m{m}^2).$

$\backslash$table$[[$Section$,1,2,3],[b(mm),200,250,300],[d(mm),750,600,500]]$

Given that: $f_c^{\text{'}}=28$MPa, and $f_y=420$MPa.

In the balanced condition, which cross$-$section is stronger and why?

$[$ Hint: find $A_s$ and $M].$