An assembly line is to be designed to operate $7(1)/(2)$ hours per day and supply a steady demand of $300$ units per day. Here are the tasks and their performance times: $[[$Task$,$Preceding Tasks,Performance Time $($Seconds$)],[$a$,-,70],[$b$,-,40],[$c$,-,45],[$d$,$a$,10],[$e$,$b$,30],[$f$,$c$,20],[$g$,$d$,60],[$h$,$e$,50],[$i$,$f$,15],[$j$,$g$,25],[$k$,$h$,$ i$,20],[$I$,$j$,$ k$,25]]$ c$.$ What is the theoretical minimum number of workstations?