\begin{table*}
%\usepackage{multirow}
\center
\begin{tabular}{|c|c|c|c|c|}
  \hline % horizontal line
  \multirow{2}{*}{1 \& 2} & \multicolumn{2}{l|}{ 3 \& 4 } & 5 & 6\\ %end line
  \cline{2-5} %short partial horizontal lines from column 2 to column 5
   & 7 & 8 & 9 & 10\\ %first cell is occupied by the multirow
  \hline
  \hline
  $ s_{1} $ & 2 & 3 & 4 & 5\  $ s_{2} $ & 2 & 3 & 4 & 5\  \hline
  \hline
  $ s_{3} $ & 2 & 3 & 4 & 5\\ 
  $ s_{4} $ & 2 & 3 & 4 & 5\  \hline
\end{tabular}
\end{table*}

\begin{table}[h]
    \caption{Optimization summary.}
    \label{tab:summary}
    \begin{tabular}{cccc}
        \toprule
        \multirow{2}{*}{Design} & \multicolumn{2}{c}{Problem size} & \multirow{2}{*}{CPU time(s)} \        \cline{2-3}
        & Binary variables & Constraints\        \midrule
        OM1 & 199 & 13,017 & 000 \        OM2 & 199 & 1,525 & 000 \        \bottomrule
    \end{tabular}
    \label{tab:my_label}
\end{table}