Supplementary angles are two angles whose measures add up to 180 degrees.
Given:
- \( m∠ABC = 15° \)
- \( m∠CBD = x° \)
Since they are supplementary, we have:
\[ m∠ABC + m∠CBD = 180° \]
Substituting the known value:
\[ 15° + x° = 180° \]
To find \( x \), we subtract 15° from both sides:
\[ x° = 180° - 15° \]
\[ x° = 165° \]
Therefore, the value of \( m∠CBD \) is 165°.