Add the following forces: 20 Newtons south, 30 newtons north, 40 newtons east, and 50 newtons west

The north and south forces have no effect on the east and west forces.

We can handle the two groups separately.

(20N south) + (30N north)

= (20N south) + (20N north + 10N north)

= 10N north.

(40N east) + (50N west)

= (40N east) + (40N west + 10N west)

= 10N west.