PROJECT CRASHING PROBLEM $($all time in Days, all Cost in $$)$: What is the Normal Duration of the project?

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

$15$

$16$

$12$

$14$

$13$