Answer:
m∠DBC = 70°
Explanation:
When an angle is bisected, it is divided into two equal parts.
Therefore, if BD bisects ∠ABC, then m∠ABD = m∠DBC.
Since ∠ABC is the sum of ∠ABD and ∠DBC, then:



Given that m∠ABC = 140°, then:

So this means that m∠DBC must be half of 140°:
