Now the same three facts, but the answer no longer falls out of a single subtraction. Four shapes of question come up again and again. A **right angle** is one of the angles given, so 90° comes off the line before anything else does — which is the one place in this topic where subtracting from 90 is right rather than wrong. An unknown is written as a **multiple**, like x and 2x, so you count the angles in units, take the given angles off, and divide. A remainder is **shared between equal angles**, so you find the leftover first and only then divide it. And sometimes the whole figure is given as a **ratio**, 4x : 7x : 9x, with no number in it anywhere. In every one of them the order matters: subtract first, divide second. Dividing the whole 180° or 360° before taking off what you were given is the mistake this lesson exists to break.
Worked example
Problem The angles at a point are 150°, x and 2x. Find x, and then find the angle marked 2x.