Vertical angles are always equal to each other. If the sum of the measures of two vertical angles is 300°, you can represent the measure of each angle as \( x \). Therefore, you can set up the following equation:
\[ x + x = 300° \]
This simplifies to:
\[ 2x = 300° \]
Now, solve for \( x \):
\[ x = \frac{300°}{2} = 150° \]
Thus, the measure of each angle is 150°.