Given the following process layout data for locating four departments $($A$,$ B$,$ C$,$ and D$)$ in four areas $(1,2,3,$ and $4)$:

$\backslash$table$[[$Distances $($meters$)],[$From $\frac{?}{T_0},1,2,3,4],[1,-,50,100,150],[2,,-,50,100],[3,,,-,50],[4,,,,]]$

Flows $($loads per week

$\backslash$table$[[$From To$,$A$,$B$,$C$,$D$],[$A$,-,10,40,50],[$B$,30,-,10,70],[$C$,60,10,-,40],[$D$,30,50,20,-]]$

What is the total flow between departments B and D$?$