Problem $6-22($Algo$)$

Francis Johnson's plant needs to design an efficient assembly line to make a new product. The assembly line needs to produce $15$ units per hour, and there is room for only four workstations. The tasks and the order in which they must be performed are shown in the following table. Tasks cannot be split, and it would be too expensive to duplicate any task.

$\backslash$table$[[$TASK$,$TASK TIME $($MINUTES$),\backslash$table$[[$IMMEDIATE$],[$PREDECESSOR$]]],[$A$,2\mathrm{.}0,-],[$B$,2\mathrm{.}0,\_],[$C$,2\mathrm{.}0,-],[$D$,1\mathrm{.}0,A,B,C$