\[
\begin{aligned}
(3721.2406)_8
&= (3 \times 8^3) + (7 \times 8^2) + (2 \times 8^1) + (1 \times 8^0) \\
&\quad + (2 \times 8^{-1}) + (4 \times 8^{-2}) + (0 \times 8^{-3}) + (6 \times 8^{-4})
\end{aligned}
\]
\[
\begin{aligned}
&= (3 \times 512) + (7 \times 64) + (2 \times 8) + (1 \times 1) \\
&\quad + (2 \times 0.125) + (4 \times 0.015625) + (0 \times 0.001953125) \\
&\quad + (6 \times 0.000244140625)
\end{aligned}
\]
\[
\begin{aligned}
&= 1536 + 448 + 16 + 1 + 0.25 + 0.0625 + 0 + 0.00146484375
\end{aligned}
\]
\[
\therefore (3721.2406)_8 = (2001.31396484375)_{10}
\]