To determine the angle measure of the cake that was left after 8 slices were eaten, we first need to calculate the total angle of the round cake. A full circle has an angle of 360 degrees.
Next, since the cake is cut into 12 equal slices, each slice will have an angle measure as follows:
\[ \text{Angle of each slice} = \frac{360^\circ}{12} = 30^\circ \]
Now, since 8 slices were eaten, we can calculate the total angle of the slices eaten:
\[ \text{Angle eaten} = 8 \times 30^\circ = 240^\circ \]
To find the angle measure of the cake that is left, we subtract the angle of the eaten slices from the total angle of the cake:
\[ \text{Angle left} = 360^\circ - 240^\circ = 120^\circ \]
Thus, the angle measure of the cake that was left is 120°.