ReasoningAges 10-12

Missing triangle angles

Use the 180° interior angle sum of a triangle to deduce an unknown angle. Review suitability for your learners before teaching.

New variation Try interactive challenge Browse templates

Age band:Ages 10-12 Ages 13-15

Choose “Save as PDF” to keep a copy. Use your browser’s paper setting for A4 or Letter.

Worksheet StudioPractice pack · Ages 10-12

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 78° and 25°. Find the third angle.
78° · 25° · Missing triangle angles
2. Two interior angles of a triangle measure 60° and 38°. Find the third angle.
60° · 38° · Missing triangle angles
3. Two interior angles of a triangle measure 27° and 60°. Find the third angle.
27° · 60° · Missing triangle angles
4. Two interior angles of a triangle measure 44° and 53°. Find the third angle.
44° · 53° · Missing triangle angles
5. Two interior angles of a triangle measure 70° and 44°. Find the third angle.
70° · 44° · Missing triangle angles
6. Two interior angles of a triangle measure 81° and 63°. Find the third angle.
81° · 63° · Missing triangle angles
7. Two interior angles of a triangle measure 60° and 53°. Find the third angle.
60° · 53° · Missing triangle angles
8. Two interior angles of a triangle measure 34° and 73°. Find the third angle.
34° · 73° · Missing triangle angles
9. Two interior angles of a triangle measure 80° and 39°. Find the third angle.
80° · 39° · Missing triangle angles
10. Two interior angles of a triangle measure 29° and 45°. Find the third angle.
29° · 45° · Missing triangle angles
11. Two interior angles of a triangle measure 65° and 52°. Find the third angle.
65° · 52° · Missing triangle angles
12. Two interior angles of a triangle measure 45° and 67°. Find the third angle.
45° · 67° · Missing triangle angles

12 questions · Seed 46 · Age guidance, not curriculum certification

View the answer key

Worksheet StudioAnswer key · Ages 10-12

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 78° and 25°. Find the third angle.
77°
Angles in a triangle sum to 180°: 180° − (78° + 25°) = 77°.
2. Two interior angles of a triangle measure 60° and 38°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (60° + 38°) = 82°.
3. Two interior angles of a triangle measure 27° and 60°. Find the third angle.
93°
Angles in a triangle sum to 180°: 180° − (27° + 60°) = 93°.
4. Two interior angles of a triangle measure 44° and 53°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (44° + 53°) = 83°.
5. Two interior angles of a triangle measure 70° and 44°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (70° + 44°) = 66°.
6. Two interior angles of a triangle measure 81° and 63°. Find the third angle.
36°
Angles in a triangle sum to 180°: 180° − (81° + 63°) = 36°.
7. Two interior angles of a triangle measure 60° and 53°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (60° + 53°) = 67°.
8. Two interior angles of a triangle measure 34° and 73°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (34° + 73°) = 73°.
9. Two interior angles of a triangle measure 80° and 39°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (80° + 39°) = 61°.
10. Two interior angles of a triangle measure 29° and 45°. Find the third angle.
106°
Angles in a triangle sum to 180°: 180° − (29° + 45°) = 106°.
11. Two interior angles of a triangle measure 65° and 52°. Find the third angle.
63°
Angles in a triangle sum to 180°: 180° − (65° + 52°) = 63°.
12. Two interior angles of a triangle measure 45° and 67°. Find the third angle.
68°
Angles in a triangle sum to 180°: 180° − (45° + 67°) = 68°.

12 questions · Seed 46 · Age guidance, not curriculum certification

Worksheet StudioPractice pack · Ages 10-12

Missing triangle angles

Take your time. Show your thinking.

1. Two interior angles of a triangle measure 78° and 25°. Find the third angle.
78° · 25° · Missing triangle angles
2. Two interior angles of a triangle measure 60° and 38°. Find the third angle.
60° · 38° · Missing triangle angles
3. Two interior angles of a triangle measure 27° and 60°. Find the third angle.
27° · 60° · Missing triangle angles
4. Two interior angles of a triangle measure 44° and 53°. Find the third angle.
44° · 53° · Missing triangle angles
5. Two interior angles of a triangle measure 70° and 44°. Find the third angle.
70° · 44° · Missing triangle angles
6. Two interior angles of a triangle measure 81° and 63°. Find the third angle.
81° · 63° · Missing triangle angles
7. Two interior angles of a triangle measure 60° and 53°. Find the third angle.
60° · 53° · Missing triangle angles
8. Two interior angles of a triangle measure 34° and 73°. Find the third angle.
34° · 73° · Missing triangle angles
9. Two interior angles of a triangle measure 80° and 39°. Find the third angle.
80° · 39° · Missing triangle angles
10. Two interior angles of a triangle measure 29° and 45°. Find the third angle.
29° · 45° · Missing triangle angles
11. Two interior angles of a triangle measure 65° and 52°. Find the third angle.
65° · 52° · Missing triangle angles
12. Two interior angles of a triangle measure 45° and 67°. Find the third angle.
45° · 67° · Missing triangle angles

12 questions · Seed 46 · Age guidance, not curriculum certification

Worksheet StudioAnswer key · Ages 10-12

Missing triangle angles

Answers and reasoning. Review before teaching.

1. Two interior angles of a triangle measure 78° and 25°. Find the third angle.
77°
Angles in a triangle sum to 180°: 180° − (78° + 25°) = 77°.
2. Two interior angles of a triangle measure 60° and 38°. Find the third angle.
82°
Angles in a triangle sum to 180°: 180° − (60° + 38°) = 82°.
3. Two interior angles of a triangle measure 27° and 60°. Find the third angle.
93°
Angles in a triangle sum to 180°: 180° − (27° + 60°) = 93°.
4. Two interior angles of a triangle measure 44° and 53°. Find the third angle.
83°
Angles in a triangle sum to 180°: 180° − (44° + 53°) = 83°.
5. Two interior angles of a triangle measure 70° and 44°. Find the third angle.
66°
Angles in a triangle sum to 180°: 180° − (70° + 44°) = 66°.
6. Two interior angles of a triangle measure 81° and 63°. Find the third angle.
36°
Angles in a triangle sum to 180°: 180° − (81° + 63°) = 36°.
7. Two interior angles of a triangle measure 60° and 53°. Find the third angle.
67°
Angles in a triangle sum to 180°: 180° − (60° + 53°) = 67°.
8. Two interior angles of a triangle measure 34° and 73°. Find the third angle.
73°
Angles in a triangle sum to 180°: 180° − (34° + 73°) = 73°.
9. Two interior angles of a triangle measure 80° and 39°. Find the third angle.
61°
Angles in a triangle sum to 180°: 180° − (80° + 39°) = 61°.
10. Two interior angles of a triangle measure 29° and 45°. Find the third angle.
106°
Angles in a triangle sum to 180°: 180° − (29° + 45°) = 106°.
11. Two interior angles of a triangle measure 65° and 52°. Find the third angle.
63°
Angles in a triangle sum to 180°: 180° − (65° + 52°) = 63°.
12. Two interior angles of a triangle measure 45° and 67°. Find the third angle.
68°
Angles in a triangle sum to 180°: 180° − (45° + 67°) = 68°.

12 questions · Seed 46 · Age guidance, not curriculum certification