A system consists of three masses: a $20kg$ mass located at the origin, a $10kg$ mass located at $(x,y)=(10,0)cm,$ and a $5kg$ mass located at $(x,y)=(0,20)cm.$ Calculate the net force acting on the $5kg$ mass. Use the R$-$r method.

For the system described above, calculate the net gravitational field at the point $(x,y)=(10,20)cm.$