Hi there!
1. Both forces are in the SAME direction, so:
25 N + 25 N = 50 N
2. The forces are in opposite directions, so:
20 N to the right (+) and 20 N to the left (-)
20 N - 20 N = 0 N
3. The forces are in opposite directions, so:
20N to the right (+) and 30 N to the left (-)
20 N - 30 N = -10 N