I am a bit confused on how the numbers $60, 40$, and $40$ came about as the flow $b, c$, and $d$ supply respectively. This example is under "Flow Networks with Supplies and Demands." I understand they added vertex a to show that multiple sources can be combined into one, but what made them choose those specific values?
Example: "Consider the network of solid edges in Figure 4.4 with supplies and demands. Vertex $b$ can supply up to $60$ units of flow, and vertices $c$ and $d$ can each supply $40$ units. Vertices $h, i$, and $j$ have flow demands of $50, 40$, and $40$ units, respectively."
The upper bounds on the arcs out of the source node $a$ are the supplies of nodes $b$, $c$, and $d$. The upper bounds on the arcs into the sink node $z$ are the demands of nodes $h$, $i$, and $j$.