Understanding that angles on a straight line add up to 180° and using this to find missing angles.
Where your child meets this in real life: Calculating angles in construction, folding paper, or design
SEAGReady breaks angles on a straight line into 3 steps, taught in order so each skill builds on the last.
supporting lesson — excluded from official-format mocks
Find the missing angle when one angle on a straight line is given
supporting lesson — excluded from official-format mocks
Find a missing angle when two or more other angles on the same straight line are given
supporting lesson — excluded from official-format mocks
Identify which angles form a straight line in a diagram with intersecting lines, then calculate missing angles
Three free sample questions from our angles on a straight line course. Every question comes with a full explanation, and hints that guide without giving the answer away.
A ray divides the straight line shown. One angle is 70 degrees. What is the other angle?
Three angles lie next to each other on the straight line shown. Two are 20 degrees and 50 degrees. What is the missing angle?
Two lines intersect as shown. One angle is 73 degrees. What is the angle directly next to it, not the opposite angle?
This is the exact interactive worked example your child sees in SEAGReady. Step through it and watch the method build up.
Grace is measuring angles where a fence post meets a wall. One angle measures 65 degrees.
What is the other angle on the straight line?
180° − 65°
Angles on a straight line add up to 180°
Step 1 of 3
Grace is measuring angles where a fence post meets a wall. One angle measures 65 degrees.
What is the other angle on the straight line?
The other angle is 115° because angles on a straight line sum to 180°.
The key insight: A straight line is half a full turn, so the angles must add to 180°!
Common mix-up: 360° − 65° = 295°. That's for angles around a point. A straight line is only 180°.
These are the misconceptions we see most often in angles on a straight line, including the ones our practice questions are specifically designed to catch.
Struggling with angles on a straight line? The real gap is often in one of these earlier topics.
SEAGReady finds the exact step where your child gets stuck, teaches it with worked examples like the one above, and brings it back for review so it sticks.
Free to start · no card needed