No diagram was provided but using the one retrieved from another source, we have a circle with a center O. Points A, B, and C are assigned around the circle. This means that ∠AOB forms a central angle. That means AB is equal to 80° as well. Now, ∠ACB forms an inscribed angle with arc AB. Recall that the value of the inscribed angle is half of the measurement of the arc. Since AB is 80°, then ∠ACB is (80/2) = 40°. Therefore, the answer is 40°.