$$a_n2^n+a_{n-1}2^{n-1} +a_{n-2}2^{n-2} +\dots +a_{0}2^{0}.$$

In binary notation we have thus $(417)_{10} =(110110001)_2$ since we have $$\begin{align*} (110100001)_2 &=1\times2^8+1\times 2^{7}+0\times 2^{6}+1\times 2^{5}+0\times 2^{4}+0\times 2^{3}\\ &+0\times 2^{2}+0\times 2^{2}+0\times 2^{1}+1\times 2^{0}. \end{align*}$$