So,
Complementary angles are angles which add together to make 90°.
To find the value of one unit in the ratio 5:7, take these steps:
Add the number of units.
5 + 7 = 12
Divide 90° by 12.
90/12 = 7.5°
The smaller angle is "worth" 5 units.
7.5 * 5 = 37.5°
