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# The coefficient of $x^5$ in the expansion of $(x+3)^6$ is

$\begin{array}{1 1}(A)\;18\\(B)\;6\\(C)\;12\\(D)\;10\end{array}$

Toolbox:
• $T_{r+1}=nC_r a^{n-r} b^r$
$(x+3)^6=(3+x)^6$
$\Rightarrow 3^6+......+6C_53x^5+3C_6x^6$
$\therefore$ Coefficient of $x^5=6C_5\times 3$
$\Rightarrow 18$
Hence (A) is the correct answer.