Based on the angle addition theorem, the measure of angle DBC is: 60°.
What is the Angle Addition Theorem?
Based on the angle addition theorem, the following equation is true:
m∠ABC = m∠ABD + m∠DBC.
Plug in the given values into the equation
75 = 15 + m∠DBC
Subtract 15 from both sides
75 - 15 = m∠DBC
m∠DBC = 60°
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