Question

The measure of an angle is 78°. What is the measure of a complementary angle?

Answer 1

First you need to know the definition of a complementary angle. That means that two angles add up to 90 degrees, or a right angle.

To find an angle complementary to a 78 degree angle, you need to subtract 78 from 90. 

90 - 78 = 12

Answer 2

Complementary angles add up to 90°.
It is given that one angle is 78°.
Let the other angle be x.
Therefore x+78=90
x=90-78
x=12°