Consider △ADBand△BDC,
BD = BD..... (common side)
AD = DC..... (given)
BA = BC..... (given)
⇒△ADB ≅ △BDC (By SSS criteria)
Consider △DBC,
⇒∠DCB=∠DAB=100∘....(congruent parts of congruent triangle)
∠DBC=30°....... (given)
∠DCB+∠CBD+∠BDC=180° ....(Angle sum property of triangle)
⇒100°+30°+∠BDC=180°
⇒130°+∠BDC=180°
⇒∠BDC=180°−130°
⇒∠BDC=50°