If c is equal to the sum of a and twice b, which of the following is the average (arithmetic mean) of a and c? A. c
B. a+b
C. a
D. b
E. a+c

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δabc ≅ δdcb by sas because

db ≅ ac . . given (side)

bc ≅ bc . . reflexive property (side)

∠dbc ≅ ∠ acb . . alternate interior angles where transversal bc crosses parallel lines bd and ac (included angle)