Concept: The question tests on the basics of Geometry
Solution
Sum of angles of a triangle = 180 degrees
Taking the smaller triangle: 2a + 120 = 180. Therefore a = 30
In triangle RST, (a + b) + (a + b) + 100 = 180
2a + 2b + 100 = 180
2b = 180 - 100 - 60 = 20
b = 10
Option E