Answer
10
Explanation
Complementary angles are angles that add up to to 90°.
In the diagram above the two angles that are complementary are 6x and (5x - 20)
6x + (5x - 20) = 90
6x + 5x - 20 = 90
11x = 90 + 20
11x = 110
x = 110/11
= 10
