∵AB=AD
∴∠ABD=∠ADB
∵∠ABD=∠C+∠CBD
∴∠ADB=∠C+∠CBD
∵∠ABC=∠ABD+∠CBD
=∠C+∠CBD+∠CBD
=∠C+2∠CBD
=∠C+30°
∴2∠CBD=30°
∴∠CBD=15°