Johnson Cogs wants to set up a line to serve $100$ customers per hour. The work elements and their precedence

$\backslash$table$[[$Work Element,Time $($sec$),$Immediate Predecessor$($s$)],[$A$,21,$None$],[$B$,20,$A$],[$C$,30,$A$],[$D$,20,$B$],[$E$,4,$B$],[$F$,15,$C$],[$G$,10,$C$],[$H$,8,$D$,$ E$],[$I$,12,$F$,$ G$],[$J$,20,$H$,$ I$],[$Total$,160,]]$

a$.$ What is the theoretical minimum number of workstations? stations. $($Enter your response rounded up to the next whole number.$)$