A kite string makes an angle of 48˚ with respect to the ground. The dashed line is vertical, and the ground is horizontal. How are the 48˚ angle and the unknown angle related? What is the value of x?

The unknown angle is 90-48=42 degrees, because the angles in a triangle must add up to 180 degrees. We already have two of them, 48 and 90, so the third angle is 180-(90+48) which is the same as (180-90)-48=90-48. The acute angles are said to be complementary.