The constant of proportionality is:
\[
\boxed{k = 1.5}
\]
The completed table is:
\[
\boxed{
\begin{tabular}{|c|c|}
\hline
Time in Minutes, \( x \) & Pieces of Bamboo, \( y \) \\
\hline
0 & 0 \\
\hline
1 & 1.5 \\
\hline
2 & 3 \\
\hline
3 & 4.5 \\
\hline
4 & 6 \\
\hline
\end{tabular}
}
\]