In the pic I attatched, the interior angles are where the dashed arc is. They are on the interior of the parallel lines. We can conclude that the A and B add up to 180 and that they are supplementary angles. Same goes for the other set of A and B. We also know that angle B = the other angle B and the same goes for A.
We know that all of those angles added together=360. If A+B=180 then add another A and B in there---
A+B+A+B=180+180
2(A+B)=360