Problem $6-15$

An assembly line is to operate eight hours per day with a desired output of $240$ units per day. The following table contains information on this product's task times and precedence relationships:

$\backslash$table$[[$TASK$,\backslash$table$[[$TASK TIME$],[($SECONDS$)]],\backslash$table$[[$IMMEDIATE$],[$PREDECESSOR$]]],[$A$,60,-],[$B$,80,$A$],[$C$,20,$A$],[$D$,50,$A$],[$E$,90,B,C$