1. **State the problem:** We are given a triangle ABC with angles at B and C measuring 70° and 30° respectively. We need to find the size of angle AC, which is the angle at vertex A. 2. **Recall the triangle angle sum property:** The sum of the interior angles of any triangle is always 180°. 3. **Calculate angle A:** Using the property, we have $$\text{angle A} + 70^\circ + 30^\circ = 180^\circ$$ 4. **Simplify the equation:** $$\text{angle A} + 100^\circ = 180^\circ$$ 5. **Solve for angle A:** $$\text{angle A} = 180^\circ - 100^\circ = 80^\circ$$ **Final answer:** The size of angle AC (angle at vertex A) is $80^\circ$.