Answer:
Using the Angle Addition Postulate, 20 + m∠DBC = 80. So, m∠DBC = 60° using the subtraction property of equality.
Step-by-step explanation:
If point D is the interior of angle ABC, then the angle addition postulate theory states that the sum of angle ABD and angle DBC is equals to angle ABC. The angle addition postulate is used to measure the resulting angle from two angles placed side by side.
From the attached image, ∠ABD and ∠DBC are placed side by side to form ∠ABC. Given that m∠ABD = 20° and m∠ABC = 80°
Hence, using angle addition postulate:
m∠ABD + m∠DBC = m∠ABC
20 + m∠DBC = 80
subtracting 20 from both sides (subtraction property of equality)
m∠DBC = 80 - 20
m∠DBC = 60°
