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°.

Here the parts are (x + 10)° and 40°, so (x + 10) + 40 = 90. Combine the numbers: x + 50 = 90, and taking 50 from both sides leaves x = 40. The labeled angle is then 40 + 10, that is 50°, and indeed 50 + 40 = 90. Checking back against the figure is the habit worth keeping.