Arc ACB is half the circle. The whole circle is 360o so ACB is 180o
Arc AC is given as 100o which leaves 80o for arc CB
There's a relationship between the size of an angle subtended by an arc of a circle.
When the angle's vertex lies on the circle the size of the angle is equal to half the arc.
So angle CAB is 40o
BTW, when the angle's vertex lies on the center of the circle, the arc and the angle are the same amount of degrees.