If m∠ABC = 80°, what is m∠DBC? Justify your reasoning.

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