Adjacent angles ABD and DBC. The measure of angle ABD is 20 degrees. If m∠ABC = 80°, what is m∠DBC? Justify your reasoning.

Using the Angle Addition Postulate, 20 + m∠DBC = 80. So, m∠DBC = 60° using the subtraction property of equality.
Using the Angle Addition Postulate, 20 + 80 = m∠DBC. So, m∠DBC = 100° using the addition property of equality.
Using the addition property of equality, 20 + 80 = 100, so m∠DBC = 100°.
Using the subtraction property of equality, 80 − 20 = 60, so m∠DBC = 60°.