Answer :

[tex]\frac{1}{3}(n+3)=6\\ n+3=18\\ n=15 \Rightarrow C[/tex]
[tex] \frac{1}{3}(n+3)=6 \\ \frac{1}{3}n+1=6 \\ -1\ \ \ \ -1 \\ \frac{1}{3}n=5 \\ n=\boxed{15} [/tex]

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