An arc subtends an angle of 80 degrees at the centre of a circle. What angle does it subtend at the circumference?

1. The same arc subtends one angle at the centre and another at the circumference. 2. The angle at the centre is twice the size of the angle at the circumference. 3. So the angle at the circumference is half of the angle at the centre. 4. Half of 80 degrees is 40 degrees. 5. The angle at the circumference is therefore 40 degrees. 6. Answering 160 degrees doubles the wrong angle, which reverses the relationship. 7. Answering 20 degrees halves the angle twice instead of once. _Source: Siyavula Mathematics Grade 11 (Everything Maths, CC BY 4.0), Chapter 8: Euclidean geometry_