AMC 10A 2011 Problem 3

Past Contests AMC 10 2011 Part A
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1

Suppose [a b] denotes the average of a and b, and {a b c} denotes the average

Answer Choices
A. \dfrac{2}{9}
B. \dfrac{5}{18}
C. \dfrac{1}{3}
D. \dfrac{7}{18}
E. \dfrac{2}{3}