To solve this, we will need to use the formulas for diameter and circumference.
First, with the radius, we can find the diameter of the circle.
D = 2r, so D = 8 inches.
Next, we can find the circumference of the entire circle.
Now, to find the measure of arc ADB, note that the angle measure is 120*. The total degree measure for a circle is 360*, so the arc is 1/3 of the circle. Therefore, the measure of the arc is