Answer:
3240 degrees.
Step-by-step explanation:
Here (as you said), this 20-gon is convex. In the image below, that is #1. One interior angle is 162 degrees. Simply multiply that by 20 to get 3240 degrees as the sum.
There are two interleaved arithmetic sequences here.
Odd terms are 5, 7, 9, 11, increasing by 2 each time.
Even terms are 1, 0, -1, decreasing by 1 each time.
The next two terms will be -2, 13.