Look at these angles in pairs (a and f, b and c, e and d).
You know that each of those pairs plus the angle between them in the inside
of the smaller triangle equals 180. So 3 sets of 180 equal 540.
Now subtract the value of the 3 angles on the inside of the triangle.
Since angles within all triangles equal 180, then 540 – 180 = 360 (C).