Question

$angle \: acb =$

Answer 1

<AOB = 80°

<ACB = 1/2 (<AOB)

= 1/2 * 80

= 40°

[ Angle subtended by a chord at centre is twice the angle subtended by the chord at Circumference of circle]

Answer 2

there is a property that angle made by a chord on the circumference is half the angle made by same chord on the center. so by this
angle ACB = angle AOB/2
angle ACB = 80/2 = 40°