PROJECT CRASHING PROBLEM $($all time in Days, all cost in $$)$: What is the EXACT SEQUENCE of activities to crash, to reduce the project by $4$ days, at least cost?

$\backslash$table$[[$Activity$,$Immediate Predecessor,Normal Time,Crash Time,Normal Cost,Crash Cost$],[$A$,-,5,4,1000,1400],[$B$,-,3,3,800,-],[$C$,$A$,$B$,3,1,600,900],[$D$,$B$,4,2,1600,2000],[$E$,$C$,$D$,5,3,800,1200],[$F$,$E$,2,1,1300,1400]]$

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