Angle on a straight line is:

## **The reasoning** Think of a straight line as a perfectly flat path — no bending, no curves. When we talk about "angles on a straight line," we mean the **total measure of angles that sit side-by-side along that line**. Imagine standing at one end of a stick and turning to face the other end. You've made a **half turn** — that's exactly **180°**. This is the fundamental property: **angles on a straight line always sum to 180°**. For example, if two angles sit on a straight line and one is 110°, the other must be 70° because 110° + 70° = 180°. ## **Why the wrong options tempt you** - **90°** catches students thinking of a right angle (one corner of a square) — but that's angles at a perpendicular, not along a straight line. - **270°** and **360°** relate to turning further: 270° is three-quarter turn, 360° is a full circle/complete turn. They're correct for *those* situations, but not for a straight line. ## **Quick takeaway** **Straight line = half turn = 180° always.** Picture flipping a ruler end-to-end — that's your 180°!