Calculate the Missing Angle in a Triangle: Step-by-Step Example
27 October 2026
If your child is learning about triangles, they will need to know how to find a missing angle when two angles are already given. This is a common type of question in the 11+ Maths: Angles in Triangles section, and it helps children practise adding and subtracting numbers with a clear purpose.
Example question
A triangle has two angles measuring 60° and 100°. Calculate the missing angle.
Step 1: Add the two known angles.
60° + 100° = 160°
Step 2: Subtract this sum from 180° (the total in a triangle).
180° - 160° = 20°
The missing angle is 20°.
After working through this example, your child will understand that the three angles in a triangle always add up to 180°. They will be ready to tackle similar problems and spot missing angles in different triangle questions.
