All of these problems use the external angle theorem which states that an exterior angle is equal to the sum of the two remote interior angles. I'll do one of the problems as an example:
#15 the exterior angle is 140 so you can set up the following equation:
140 = (3x + 4) + (8x + 4) simplifying the right hand side
140 = 11x + 8 subtracting 8 from both sides
132 = 11x dividing both sides by 11
12 = x now if x = 12 angle S is 3(12) + 4 = 40