**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**