PROJECT CRASHING PROBLEM $($all time in Days, all Cost in $$)$: What is the total crashing cost to crash the project by $4$ days?

$\backslash$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$,2,1,1300,1400]]$

$$750$

$$800$

$$650$

$$600$

$$700$