Answer: 30º. The key here is that the triangle includes parallel lines.
Remember the congruent angles involved with parallel lines? Sure you do. So
think of them when looking at the picture: Angle B must be the same as angle D,
namely 80º. Which brings us to:
Similar Triangle Rule: The top triangle is similar to the big triangle if their
bases are parallel.
Got it? So if you see a triangle intersected by a line parallel to its base, the
new, smaller triangle is similar to the larger.