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.
Find the missing angle when one angle on a straight line is given
Find a missing angle when two or more other angles on the same straight line are given
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.
Sean is looking at where a signpost meets the ground. One angle measures 70 degrees. What is the other angle on the straight line?
Answer: A. 110 degrees
Angles on a straight line add up to 180 degrees. 180 - 70 = 110 degrees The other angle is 110 degrees.
Stuck? Start here: What do angles on a straight line add up to?
Three angles are on a straight line. Two of them are 40 degrees and 55 degrees. What is the third angle?
Answer: C. 85 degrees
Step 1: Add the known angles. 40 + 55 = 95 degrees Step 2: Angles on a straight line sum to 180 degrees. 180 - 95 = 85 degrees The third angle is 85 degrees.
Stuck? Start here: First, add the two known angles together.
Two lines cross each other. One of the angles where they meet is 105 degrees. What is the angle next to it on the same straight line?
Answer: A. 75 degrees
Adjacent angles on a straight line add up to 180 degrees. 180 - 105 = 75 degrees The angle next to the 105 degree angle is 75 degrees.
Stuck? Start here: Look at two adjacent angles that sit on one straight line.
This is the exact interactive worked example your child sees in SEAGReady. Step through it and watch the method build up.
Ciara 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°
Step 1 of 3
Ciara 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°!
Watch out: 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.