Two angles are complementary if they sum to 90 degrees.
Let `x` be the angle, then its complementary angle is `90-x` . We are told that the angle is half as large as its complement, so
`x=1/2(90-x)` multiply by 2
`2x=90-x` move x to left side
`3x=90` divide by 3
The angle is 30 degrees.