Question

if m∠1 = 120°, what is the m∠3?

Explanation:

Step1: Identify angle - pair relationship

$\angle1$ and $\angle2$ are a linear - pair.

Step2: Calculate m$\angle2$

Since $\angle1+\angle2 = 180^{\circ}$ and $\angle1 = 120^{\circ}$, then $\angle2=180^{\circ}-120^{\circ}=60^{\circ}$.

Step3: Identify congruent angles

$\angle2$ and $\angle3$ are alternate - interior angles.

Step4: Determine m$\angle3$

Alternate - interior angles are congruent, so $m\angle3 = m\angle2=60^{\circ}$.

Answer:

$60^{\circ}$