Angie and Becky each completed a separate proof to show that the measures of vertical angles AKG and HKB are equal. Who completed the proof incorrectly? Explain.

line AB intersecting line GH at K

Angie's Proof

Statement Justification
1. Segment GH intersects segment AB at K 1. Given
2. m∠AKG + m∠GKB = 180°
m∠GKB + m∠HKB = 180° 2. Definition of Supplementary Angles
3. m∠AKG + m∠ GKB = m∠GKB + m∠HKB 3. Substitution Property
4. m∠AKG = m∠HKB 4. Subtraction Property

Becky's Proof

Statement Justification
1. Segment GH intersects segment AB at K 1. Given
2. m∠AKG + m∠GKB = 180°
m∠GKB + m∠HKB = 180° 2. Angle Addition Postulate
3. m∠AKG + m∠ GKB = m∠GKB + m∠HKB 3. Substitution Property
4. m∠AKG = m∠HKB 4. Subtraction Property

The only difference of the two proofs, at least what I can see, is the reason for step 2. Angie claims that it is the Definition of Supplementary Angles, while Becky claims Angle Addition Postulate.

The Definition of Supplementary Angles states that m∠A + m∠B = 180°.

While the Angle Addition Postulate states that m∠A + m∠B = m∠C.

Because the context used is that m∠AKG + m∠GKB = 180° and m∠GKB + m∠HKB = 180°, it would be using the Definition of Supplementary Angles.

So Becky is the one who is incorrect.