Answer: +20N
Explanation:
First we look at our given information. You are finding the sum of the forces on the ball and they already give you the magnitude and direction of the forces. This means we can disregard the mass (1kg) of the ball and what the forces are from (gravity and floor pushing on the ball). The only things that matter are magnitude and direction.
Now we have to give each force either a positive or negative sign. I am assuming we are using the ground as a frame of reference. When we move towards a frame of reference, the sign is negative. When we move away from a frame of reference. This means that the upward vector will have a positive sign (+30N) and the downward vector will have a negative sign (-10N).
Now all we have to do is add the two together:
+30 + (-10) = +20
All done!!