1. 3,240° (see steps below)
(S-2) x 180 = A
(20-2) x 180 = A
(18) x 180 = A
3,240 = A

2. 5,040° (see steps below)
(S-2) x 180 = A
(30-2) x 180 = A
(28) x 180 = A
5,040 = A

3. 225,000° (see steps below)
(S-2) x 180 = A
(1,252-2) x 180 = A
(1,250) x 180 = A
225,000 = A

4. 360°
Why? Because you should know that the rule is:
All polygon's sums of their exterior angles is 360°!