Explain

Here's the way I think of it if it helps:

999₁₆   =   9(16⁰)   +   9(16¹)  +  9(16²)

999₁₆   =   9(1)   +   9(16)  +  9(256)

999₁₆   =   9  +  144  +  2304

999₁₆   =   2457     that is in base 10

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Convert $999_{16}$ to decimal

$\text{Let ${\color{red}9}{\color{blue}9}{\color{green}9} $}$

$\begin{array}{|rcll|} \hline {\color{red}9}\cdot 16 &=& 144 \\ 144 + {\color{blue}9} &=& 153 \\ 153 \cdot 16 &=& 2448 \\ 2448 + {\color{green}9} &=& 2457 \\ \hline \end{array}$

$\mathbf{999_{16} = 2457_{10} }$