Geometry

Question Analysis：
This question uses algebraic expressions for two adjacent angles in a parallelogram.
The main focus is on the supplementary relationship of adjacent angles.

Key Concept Explanation：
Adjacent angles in a parallelogram add up to 180 degrees. So, angle P + angle Q = 180.

Step-by-Step Solution：
Add the expressions: (2x + 20) + 6x = 180
Simplify: 8x + 20 = 180
Subtract 20: 8x = 160
Divide by 8: x = 20

Common Mistakes：
Forgetting that adjacent angles are supplement...