Can you Explain how we draw them?

Example 1 \begin{tabular}{|l|l|l|} \hline ActivityName & Predecessors & Duration \\ \hline A & - & 3 \\ \hline B & - & 2 \\ \hline C & A & 3 \\ \hline D & A & 7 \\ \hline E & B & 5 \\ \hline F & C & 6 \\ \hline G & D,E & 5 \\ \hline \end{tabular} \begin{tabular}{|c|c|c|} \hline \hline Activity & Predecessor & Duration \\ \hline A & - & 4 \\ \hline B & A & 7 \\ \hline C & A & 5 \\ \hline D & B & 8 \\ \hline E & B, C & 5 \\ \hline F & C & 2 \\ \hline G & D, E , F & 1 \\ \hline \end{tabular}