LaTeX tables

\begin{table}[htbp]
\centering % this centres the whole of the table.
\begin{tabular}{ccc}
\toprule % new command, using booktabs
\multicolumn{2}{c}{two cols} & {one col} \\
$u_{x}\,(ms^{-1})$ & $u_{mean_{x, y, z}}\, (ms^{-1})$ & $\frac{u_{mean_{x}}}{u_{x}}$ \\
\midrule
10 & 7.4, 6E-5, 1E-4 & 0.74\\
20 & 14.9, 1E-4, 5E-4 & 0.75\\
30 & 23.5, 9E-5, 8E-4 & 0.78\\
40 & 32.3, 2E-4, 1E-3 & 0.81\\
50 & 40.0, 3E-4, 2E-2 & 0.80\\
\bottomrule
\end{tabular}
\caption{Effect of changing from uniform to steady, non-uniform flow, on the
mean velocity in the duct.}
\label{tab:uniform-to-steady-non-uniform-vels}
\end{table}