Finding the Complement

Figures often label an angle with an expression instead of a bare number: an angle of (x + 10)° says “10 more than the unknown”. The rules do not change. If the parts fill a right angle, they still add to 90°.

The two angles are (x + 10) degrees and 40°, so (x + 10) + 40 = 90. Combine the constants to get x + 50 = 90, then subtract 50 to find x = 40. The labeled angle is therefore 50°, and 50 + 40 = 90 confirms the right angle.