Use angles on a line, around a point, in a triangle and in a quadrilateral to find missing angles — and give the reason each time. It is a Foundation topic, aimed at grade 2. Free questions with answers are on this page.
A handful of facts unlock almost every angle question at this grade. Angles on a straight line add to 180∘. Angles around a point add to 360∘. Angles in a triangle add to 180∘; in a quadrilateral, 360∘. Vertically opposite angles (the ones that make an X) are equal. An equilateral triangle has three 60∘ angles; an isosceles triangle has two equal base angles.
The method is always: write the fact, write the sum, solve for the missing angle. Fourth angle of a quadrilateral with 85∘, 110∘ and 72∘ is 360−267=93∘. Reasons must be written in the accepted phrases — 'angles in a quadrilateral add to 360∘' — because the reason is often a mark of its own.
Do not assume a triangle is isosceles or a line is straight unless the diagram is marked or the question says so. A picture that 'looks like' 90∘ is not evidence.
Objectives
A full turn is 360°. A straight line is 180°.
Angles that meet at a point add to 360°.
Two angles that make a straight line add to 180°.
Key points
Find a missing angle on a straight line, around a point, in a triangle and in a quadrilateral.
Use the properties of isosceles and equilateral triangles.
Write the correct reason for each step in the standard wording.
Chain two or more facts together to reach an angle that is not directly given.
Worked examples
Example 1
Two angles on a straight line are 25° and an unknown angle. What is the unknown angle?
2 marks
Angles on a straight line add to 180°, and one angle is 25°.
Subtract: 180 − 25 = 155.
The unknown angle is 155°.
Answer: 155°
Common slips
Slip: Angles on a straight line add to 182°, and one angle is 25°. That changes a number from the question, so it answers a different question.
Slip: Subtract: 183 − 25 = 155. That changes a number in the working, so the step no longer follows from the one before.
Slip: The unknown angle is 159°. That does not match the working above it.
Example 2
A triangle has angles 26° and 30°. What is the third angle?
2 marks
The three angles in a triangle add to 180°. The known angles add to 26 + 30 = 56.
Subtract: 180 − 56 = 124.
The third angle is 124°.
Answer: 124°
Common slips
Slip: The three angles in a triangle add to 182°. The known angles add to 26 + 30 = 56. That changes a number from the question, so it answers a different question.
Slip: Subtract: 183 − 56 = 124. That changes a number in the working, so the step no longer follows from the one before.
Slip: The third angle is 128°. That does not match the working above it.
Example 3
Three angles meet at a point and fill a full turn. Two of them are 90° and 100°. What is the third angle?
2 marks
Angles around a point add to 360°. The known angles add to 90 + 100 = 190.
Subtract: 360 − 190 = 170.
The third angle is 170°.
Answer: 170°
Common slips
Slip: Angles around a point add to 362°. The known angles add to 90 + 100 = 190. That changes a number from the question, so it answers a different question.
Slip: Subtract: 363 − 190 = 170. That changes a number in the working, so the step no longer follows from the one before.
Slip: The third angle is 174°. That does not match the working above it.
Questions and answers
GCSE Maths angles questions and answers. Try each one, then open the answer. Easy questions come first.
Question 1 Easy2 marks
Two angles of a triangle are 40° and 60°. What is the third angle?
Show answer
Answer: 80
Angles in a triangle add to 180°.
The value is 80.
M1 — Angles in a triangle add to 180°.
A1 — Answer 80
Question 2 Easy2 marks
A straight line is split into 50° and another angle. What is the other angle?
Show answer
Answer: 130
Angles on a straight line add to 180°.
The value is 130.
M1 — Angles on a straight line add to 180°.
A1 — Answer 130
Question 3 Medium2 marks
Three angles of a quadrilateral are 80°, 90° and 70°. What is the fourth?
Show answer
Answer: 120
Angles in a quadrilateral add to 360°.
The value is 120.
M1 — Angles in a quadrilateral add to 360°.
A1 — Answer 120
Question 4 Medium2 marks
An isosceles triangle has a vertex angle of 40°. What is one base angle?
Show answer
Answer: 70
The two base angles share what is left from 180°.
The value is 70.
M1 — The two base angles share what is left from 180°.
A1 — Answer 70
Question 5 Hard2 marks
A pair of co-interior angles meet a transversal. One is 70°. What is the other?
Show answer
Answer: 110
Co-interior angles add to 180°.
The value is 110.
M1 — Co-interior angles add to 180°.
A1 — Answer 110
Question 6 Hard2 marks
A regular pentagon has an exterior angle of 72°. One interior angle sits next to it on a straight line. What is that interior angle?
Show answer
Answer: 108
The interior and exterior angles on a straight line add to 180°.
The value is 108.
M1 — The interior and exterior angles on a straight line add to 180°.
A1 — Answer 108
Question 7 Exam-style2 marks
A pair of co-interior angles meet a transversal. One is 71°. What is the other?
Show answer
Answer: 109
Co-interior angles add to 180°.
The value is 109.
M1 — Co-interior angles add to 180°.
A1 — Answer 109
Common mistakes
Most lost marks on this topic are one slip.
Using 360° for a straight line.
A straight line is 180°.
Adding the triangle angles to 360°.
A triangle sums to 180°.
Treating vertically opposite angles as angles that add to 180°.
Opposite angles are equal. Adjacent ones add to 180°.
Spot the slip
Angles on a straight line add to
180°
90°
360°
Show the slip
180° A straight line is half a full turn.
Exam tips
Be ready to do this without a calculator. Paper 1 is the non-calculator paper.
If the question says “show that”, the result is given. The marks are for the steps that reach it.
The method is the same on AQA-style and Edexcel-style papers.
If the question says 'give a reason', there is a mark for the words. Use the standard phrases exactly.
Write each new angle on the diagram as you find it; it stops you losing track in a chain of steps.
Printable worksheet
The same questions are in a PDF you can print. Answers start on a later page.