Analysis of double$-$reinforced rectangular beam

There is a simply supported beam bearing a concentrated load $P$ at $\frac{L}{3}$ and $2\frac{L}{3},$ as shown in the figure below, where $L=6m,$ th

concentrated load is divided into static loads $PD=15kN,PL=20kN,WD=20k\frac{N}{m},WL=30k\frac{N}{m},$ please draw:

$(1)$ The bending moment diagram when the beam is subjected to the ultimate load Pu and W $,$ and find the ultimate bending moment $(20\%)$ of the critical section