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 .
