1. The problem is to understand how to determine that the third angle in a triangle is 30 degrees. 2. The key rule is that the sum of the interior angles in any triangle is always 180 degrees. 3. If you know two angles, you can find the third by subtracting the sum of the known angles from 180. 4. The formula is: $$\text{Third angle} = 180 - (\text{Angle 1} + \text{Angle 2})$$ 5. For example, if the first two angles are 70 and 80 degrees, then: $$\text{Third angle} = 180 - (70 + 80) = 180 - 150 = 30$$ 6. This shows the third angle is 30 degrees because the total must be 180 degrees. 7. This method applies to any triangle when two angles are known.