【Answer】113
【Analysis】Exam point: Periodic problem. Observing the table, we find that every 4 columns form a cycle, with a total of 9 numbers in each cycle. $51 \div 4=12$ (groups).....3 (columns), $12 \times 9=108$, so the number in the third row of the 51st column is $108+5=113$.
13. As shown in the figure, natural numbers starting from 1 are arranged according to a certain rule. What is the number in the 3rd row and 51st column?
$\qquad$
\begin{tabular}{|l|l|l|l|l|l|l|l|l|l|}
\hline 3 & & 7 & 9 & 12 & & 16 & 18 & 21 & $\ldots$ \\
\hline 2 & 4 & 6 & & 11 & 13 & 15 & & 20 & $\ldots$ \\
\hline 1 & & 5 & 8 & 10 & & 14 & 17 & 19 & $\ldots$ \\
\hline
\end{tabular}