$4\mathrm{.}19$ The forces $F_A=30lb,F_B=40lb,F_C=20lb,$ and $F_D=30lb.$ What is the sum of the moments of the forces about the origin of the coordinate system?

$4\mathrm{.}20$ The force $F_A=30lb.$ The vector sum of the forces on the beam is zero, and the sum of the moments of the forces about the origin of the coordinate system is zero. $($a$)$ Determine the forces $F_B,F_C,$ and $F_D.($b$)$ Determine the sum of the moments of the forces about the right end of the beam.