Problem $1)$ A beam of cross section shown is made of steel which is assumed to be plasto$-$plastic.

a$)$ Determine the plastic moment $(M_B)$

The material yield stress $=250$MPa.

$($I See this answered but noticed outer area is $40$x$80$mm$,$ the previous answer shows $40$x$40$mm$.$ what is the reason for this?$)$